What is This Library?
This library provides functionality to send internet mail via SMTP, the Simple Mail Transfer Protocol. For details of SMTP itself, see [RFC2821] (www.ietf.org/rfc/rfc2821.txt).
What is This Library NOT?
This library does NOT provide functions to compose internet mails. You must create them by yourself. If you want better mail support, try RubyMail or TMail or search for alternatives in RubyGems.org or The Ruby Toolbox.
FYI: the official documentation on internet mail is: [RFC2822] (www.ietf.org/rfc/rfc2822.txt).
Examples
Sending Messages
You must open a connection to an SMTP server before sending messages. The first argument is the address of your SMTP server, and the second argument is the port number. Using SMTP.start with a block is the simplest way to do this. This way, the SMTP connection is closed automatically after the block is executed.
require 'net/smtp' Net::SMTP.start('your.smtp.server', 25) do |smtp| # Use the SMTP object smtp only in this block. end
Replace ‘your.smtp.server’ with your SMTP server. Normally your system manager or internet provider supplies a server for you.
Then you can send messages.
msgstr = <<END_OF_MESSAGE From: Your Name <your@mail.address> To: Destination Address <someone@example.com> Subject: test message Date: Sat, 23 Jun 2001 16:26:43 +0900 Message-Id: <unique.message.id.string@example.com> This is a test message. END_OF_MESSAGE require 'net/smtp' Net::SMTP.start('your.smtp.server', 25) do |smtp| smtp.send_message msgstr, 'your@mail.address', 'his_address@example.com' end
Closing the Session
You MUST close the SMTP session after sending messages, by calling the finish method:
# using SMTP#finish smtp = Net::SMTP.start('your.smtp.server', 25) smtp.send_message msgstr, 'from@address', 'to@address' smtp.finish
You can also use the block form of SMTP.start/SMTP#start. This closes the SMTP session automatically:
# using block form of SMTP.start Net::SMTP.start('your.smtp.server', 25) do |smtp| smtp.send_message msgstr, 'from@address', 'to@address' end
I strongly recommend this scheme. This form is simpler and more robust.
HELO domain
In almost all situations, you must provide a third argument to SMTP.start/SMTP#start. This is the domain name which you are on (the host to send mail from). It is called the “HELO domain”. The SMTP server will judge whether it should send or reject the SMTP session by inspecting the HELO domain.
Net::SMTP.start('your.smtp.server', 25,
'mail.from.domain') { |smtp| ... }
SMTP Authentication
The Net::SMTP class supports three authentication schemes; PLAIN, LOGIN and CRAM MD5. (SMTP Authentication: [RFC2554]) To use SMTP authentication, pass extra arguments to SMTP.start/SMTP#start.
# PLAIN Net::SMTP.start('your.smtp.server', 25, 'mail.from.domain', 'Your Account', 'Your Password', :plain) # LOGIN Net::SMTP.start('your.smtp.server', 25, 'mail.from.domain', 'Your Account', 'Your Password', :login) # CRAM MD5 Net::SMTP.start('your.smtp.server', 25, 'mail.from.domain', 'Your Account', 'Your Password', :cram_md5)
Authentication
Seconds to wait while attempting to open a connection. If the connection cannot be opened within this time, a Net::OpenTimeout is raised. The default value is 30 seconds.
Seconds to wait while reading one block (by one read(2) call). If the read(2) call does not complete within this time, a Net::ReadTimeout is raised. The default value is 60 seconds.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 175
def SMTP.default_port
25
end
The default SMTP port number, 25.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 193
def SMTP.default_ssl_context
OpenSSL::SSL::SSLContext.new
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 180
def SMTP.default_submission_port
587
end
The default mail submission port number, 587.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 185
def SMTP.default_tls_port
465
end
The default SMTPS port number, 465.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 208
def initialize(address, port = nil)
@address = address
@port = (port || SMTP.default_port)
@esmtp = true
@capabilities = nil
@socket = nil
@started = false
@open_timeout = 30
@read_timeout = 60
@error_occurred = false
@debug_output = nil
@tls = false
@starttls = false
@ssl_context = nil
end
Creates a new Net::SMTP object.
address is the hostname or ip address of your SMTP server. port is the port to connect to; it defaults to port 25.
This method does not open the TCP connection. You can use SMTP.start instead of SMTP.new if you want to do everything at once. Otherwise, follow SMTP.new with SMTP#start.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 452
def SMTP.start(address, port = nil, helo = 'localhost',
user = nil, secret = nil, authtype = nil,
&block) # :yield: smtp
new(address, port).start(helo, user, secret, authtype, &block)
end
Creates a new Net::SMTP object and connects to the server.
This method is equivalent to:
Net::SMTP.new(address, port).start(helo_domain, account, password, authtype)
Example
Net::SMTP.start('your.smtp.server') do |smtp| smtp.send_message msgstr, 'from@example.com', ['dest@example.com'] end
Block Usage
If called with a block, the newly-opened Net::SMTP object is yielded to the block, and automatically closed when the block finishes. If called without a block, the newly-opened Net::SMTP object is returned to the caller, and it is the caller’s responsibility to close it when finished.
Parameters
address is the hostname or ip address of your smtp server.
port is the port to connect to; it defaults to port 25.
helo is the HELO domain provided by the client to the server (see overview comments); it defaults to ‘localhost’.
The remaining arguments are used for SMTP authentication, if required or desired. user is the account name; secret is your password or other authentication token; and authtype is the authentication type, one of :plain, :login, or :cram_md5. See the discussion of SMTP Authentication in the overview notes.
Errors
This method may raise:
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 271
def auth_capable?(type)
return nil unless @capabilities
return false unless @capabilities['AUTH']
@capabilities['AUTH'].include?(type)
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 751
def auth_cram_md5(user, secret)
check_auth_args user, secret
res = critical {
res0 = get_response('AUTH CRAM-MD5')
check_auth_continue res0
crammed = cram_md5_response(secret, res0.cram_md5_challenge)
get_response(base64_encode("#{user} #{crammed}"))
}
check_auth_response res
res
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 740
def auth_login(user, secret)
check_auth_args user, secret
res = critical {
check_auth_continue get_response('AUTH LOGIN')
check_auth_continue get_response(base64_encode(user))
get_response(base64_encode(secret))
}
check_auth_response res
res
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 771
def auth_method(type)
"auth_#{type.to_s.downcase}".intern
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 731
def auth_plain(user, secret)
check_auth_args user, secret
res = critical {
get_response('AUTH PLAIN ' + base64_encode("\0#{user}\0#{secret}"))
}
check_auth_response res
res
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 725
def authenticate(user, secret, authtype = DEFAULT_AUTH_TYPE)
check_auth_method authtype
check_auth_args user, secret
send auth_method(authtype), user, secret
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 784
def base64_encode(str)
# expects "str" may not become too long
[str].pack('m0')
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 247
def capable?(key)
return nil unless @capabilities
@capabilities[key] ? true : false
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 280
def capable_auth_types
return [] unless @capabilities
return [] unless @capabilities['AUTH']
@capabilities['AUTH']
end
Returns supported authentication methods on this server. You cannot get valid value before opening SMTP session.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 267
def capable_cram_md5_auth?
auth_capable?('CRAM-MD5')
end
true if server advertises AUTH CRAM-MD5. You cannot get valid value before opening SMTP session.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 261
def capable_login_auth?
auth_capable?('LOGIN')
end
true if server advertises AUTH LOGIN. You cannot get valid value before opening SMTP session.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 255
def capable_plain_auth?
auth_capable?('PLAIN')
end
true if server advertises AUTH PLAIN. You cannot get valid value before opening SMTP session.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 243
def capable_starttls?
capable?('STARTTLS')
end
true if server advertises STARTTLS. You cannot get valid value before opening SMTP session.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 775
def check_auth_args(user, secret, authtype = DEFAULT_AUTH_TYPE)
unless user
raise ArgumentError, 'SMTP-AUTH requested but missing user name'
end
unless secret
raise ArgumentError, 'SMTP-AUTH requested but missing secret phrase'
end
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 985
def check_auth_continue(res)
unless res.continue?
raise res.exception_class, res.message
end
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 765
def check_auth_method(type)
unless respond_to?(auth_method(type), true)
raise ArgumentError, "wrong authentication type #{type}"
end
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 979
def check_auth_response(res)
unless res.success?
raise SMTPAuthenticationError, res.message
end
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 973
def check_continue(res)
unless res.continue?
raise SMTPUnknownError, "could not get 3xx (#{res.status}: #{res.string})"
end
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 967
def check_response(res)
unless res.success?
raise res.exception_class, res.message
end
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 793
def cram_md5_response(secret, challenge)
tmp = Digest::MD5.digest(cram_secret(secret, IMASK) + challenge)
Digest::MD5.hexdigest(cram_secret(secret, OMASK) + tmp)
end
CRAM-MD5: [RFC2195]
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 800
def cram_secret(secret, mask)
secret = Digest::MD5.digest(secret) if secret.size > CRAM_BUFSIZE
buf = secret.ljust(CRAM_BUFSIZE, "\0")
0.upto(buf.size - 1) do |i|
buf[i] = (buf[i].ord ^ mask).chr
end
buf
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 957
def critical
return Response.parse('200 dummy reply code') if @error_occurred
begin
return yield()
rescue Exception
@error_occurred = true
raise
end
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 891
def data(msgstr = nil, &block) #:yield: stream
if msgstr and block
raise ArgumentError, "message and block are exclusive"
end
unless msgstr or block
raise ArgumentError, "message or block is required"
end
res = critical {
check_continue get_response('DATA')
socket_sync_bak = @socket.io.sync
begin
@socket.io.sync = false
if msgstr
@socket.write_message msgstr
else
@socket.write_message_by_block(&block)
end
ensure
@socket.io.flush
@socket.io.sync = socket_sync_bak
end
recv_response()
}
check_response res
res
end
This method sends a message. If msgstr is given, sends it as a message. If block is given, yield a message writer stream. You must write message before the block is closed.
# Example 1 (by string) smtp.data(<<EndMessage) From: john@example.com To: betty@example.com Subject: I found a bug Check vm.c:58879. EndMessage # Example 2 (by block) smtp.data {|f| f.puts "From: john@example.com" f.puts "To: betty@example.com" f.puts "Subject: I found a bug" f.puts "" f.puts "Check vm.c:58879." }
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 393
def debug_output=(arg)
@debug_output = arg
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 351
def disable_starttls
@starttls = false
@ssl_context = nil
end
Disables SMTP/TLS (STARTTLS) for this object. Must be called before the connection is established to have any effect.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 307
def disable_tls
@tls = false
@ssl_context = nil
end
Disables SMTP/TLS for this object. Must be called before the connection is established to have any effect.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 611
def do_finish
quit if @socket and not @socket.closed? and not @error_occurred
ensure
@started = false
@error_occurred = false
@socket.close if @socket
@socket = nil
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 599
def do_helo(helo_domain)
res = @esmtp ? ehlo(helo_domain) : helo(helo_domain)
@capabilities = res.capabilities
rescue SMTPError
if @esmtp
@esmtp = false
@error_occurred = false
retry
end
raise
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 542
def do_start(helo_domain, user, secret, authtype)
raise IOError, 'SMTP session already started' if @started
if user or secret
check_auth_method(authtype || DEFAULT_AUTH_TYPE)
check_auth_args user, secret
end
s = Timeout.timeout(@open_timeout, Net::OpenTimeout) do
tcp_socket(@address, @port)
end
logging "Connection opened: #{@address}:#{@port}"
@socket = new_internet_message_io(tls? ? tlsconnect(s) : s)
check_response critical { recv_response() }
do_helo helo_domain
if starttls_always? or (capable_starttls? and starttls_auto?)
unless capable_starttls?
raise SMTPUnsupportedCommand,
"STARTTLS is not supported on this server"
end
starttls
@socket = new_internet_message_io(tlsconnect(s))
# helo response may be different after STARTTLS
do_helo helo_domain
end
authenticate user, secret, (authtype || DEFAULT_AUTH_TYPE) if user
@started = true
ensure
unless @started
# authentication failed, cancel connection.
s.close if s
@socket = nil
end
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 829
def ehlo(domain)
getok("EHLO #{domain}")
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 333
def enable_starttls(context = SMTP.default_ssl_context)
raise 'openssl library not installed' unless defined?(OpenSSL)
raise ArgumentError, "SMTPS and STARTTLS is exclusive" if @tls
@starttls = :always
@ssl_context = context
end
Enables SMTP/TLS (STARTTLS) for this object. context is a OpenSSL::SSL::SSLContext object.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 342
def enable_starttls_auto(context = SMTP.default_ssl_context)
raise 'openssl library not installed' unless defined?(OpenSSL)
raise ArgumentError, "SMTPS and STARTTLS is exclusive" if @tls
@starttls = :auto
@ssl_context = context
end
Enables SMTP/TLS (STARTTLS) for this object if server accepts. context is a OpenSSL::SSL::SSLContext object.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 296
def enable_tls(context = SMTP.default_ssl_context)
raise 'openssl library not installed' unless defined?(OpenSSL)
raise ArgumentError, "SMTPS and STARTTLS is exclusive" if @starttls
@tls = true
@ssl_context = context
end
Enables SMTP/TLS (SMTPS: SMTP over direct TLS connection) for this object. Must be called before the connection is established to have any effect. context is a OpenSSL::SSL::SSLContext object.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 531
def finish
raise IOError, 'not yet started' unless started?
do_finish
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 941
def get_response(reqline)
validate_line reqline
@socket.writeline reqline
recv_response()
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 931
def getok(reqline)
validate_line reqline
res = critical {
@socket.writeline reqline
recv_response()
}
check_response res
res
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 825
def helo(domain)
getok("HELO #{domain}")
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 225
def inspect
"#<#{self.class} #{@address}:#{@port} started=#{@started}>"
end
Provide human-readable stringification of class state.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 1070
def logging(msg)
@debug_output << msg + "\n" if @debug_output
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 833
def mailfrom(from_addr)
if $SAFE > 0
raise SecurityError, 'tainted from_addr' if from_addr.tainted?
end
getok("MAIL FROM:<#{from_addr}>")
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 594
def new_internet_message_io(s)
InternetMessageIO.new(s, read_timeout: @read_timeout,
debug_output: @debug_output)
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 709
def open_message_stream(from_addr, *to_addrs, &block) # :yield: stream
raise IOError, 'closed session' unless @socket
mailfrom from_addr
rcptto_list(to_addrs) {data(&block)}
end
Opens a message writer stream and gives it to the block. The stream is valid only in the block, and has these methods:
- puts(str = ”)
-
outputs STR and CR LF.
- print(str)
-
outputs STR.
- printf(fmt, *args)
-
outputs sprintf(fmt,*args).
- write(str)
-
outputs STR and returns the length of written bytes.
- <<(str)
-
outputs STR and returns self.
If a single CR (“r”) or LF (“n”) is found in the message, it is converted to the CR LF pair. You cannot send a binary message with this method.
Parameters
from_addr is a String representing the source mail address.
to_addr is a String or Strings or Array of Strings, representing the destination mail address or addresses.
Example
Net::SMTP.start('smtp.example.com', 25) do |smtp| smtp.open_message_stream('from@example.com', ['dest@example.com']) do |f| f.puts 'From: from@example.com' f.puts 'To: dest@example.com' f.puts 'Subject: test message' f.puts f.puts 'This is a test message.' end end
Errors
This method may raise:
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 918
def quit
getok('QUIT')
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 861
def rcptto(to_addr)
if $SAFE > 0
raise SecurityError, 'tainted to_addr' if to_addr.tainted?
end
getok("RCPT TO:<#{to_addr}>")
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 840
def rcptto_list(to_addrs)
raise ArgumentError, 'mail destination not given' if to_addrs.empty?
ok_users = []
unknown_users = []
to_addrs.flatten.each do |addr|
begin
rcptto addr
rescue SMTPAuthenticationError
unknown_users << addr.dump
else
ok_users << addr
end
end
raise ArgumentError, 'mail destination not given' if ok_users.empty?
ret = yield
unless unknown_users.empty?
raise SMTPAuthenticationError, "failed to deliver for #{unknown_users.join(', ')}"
end
ret
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 374
def read_timeout=(sec)
@socket.read_timeout = sec if @socket
@read_timeout = sec
end
Set the number of seconds to wait until timing-out a read(2) call.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 947
def recv_response
buf = ''.dup
while true
line = @socket.readline
buf << line << "\n"
break unless line[3,1] == '-' # "210-PIPELINING"
end
Response.parse(buf)
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 817
def rset
getok('RSET')
end
Aborts the current mail transaction
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 656
def send_message(msgstr, from_addr, *to_addrs)
raise IOError, 'closed session' unless @socket
mailfrom from_addr
rcptto_list(to_addrs) {data msgstr}
end
Sends msgstr as a message. Single CR (“r”) and LF (“n”) found in the msgstr, are converted into the CR LF pair. You cannot send a binary message with this method. msgstr should include both the message headers and body.
from_addr is a String representing the source mail address.
to_addr is a String or Strings or Array of Strings, representing the destination mail address or addresses.
Example
Net::SMTP.start('smtp.example.com') do |smtp| smtp.send_message msgstr, 'from@example.com', ['dest@example.com', 'dest2@example.com'] end
Errors
This method may raise:
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 575
def ssl_socket(socket, context)
OpenSSL::SSL::SSLSocket.new socket, context
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 514
def start(helo = 'localhost',
user = nil, secret = nil, authtype = nil) # :yield: smtp
if block_given?
begin
do_start helo, user, secret, authtype
return yield(self)
ensure
do_finish
end
else
do_start helo, user, secret, authtype
return self
end
end
Opens a TCP connection and starts the SMTP session.
Parameters
helo is the HELO domain that you’ll dispatch mails from; see the discussion in the overview notes.
If both of user and secret are given, SMTP authentication will be attempted using the AUTH command. authtype specifies the type of authentication to attempt; it must be one of :login, :plain, and :cram_md5. See the notes on SMTP Authentication in the overview.
Block Usage
When this methods is called with a block, the newly-started SMTP object is yielded to the block, and automatically closed after the block call finishes. Otherwise, it is the caller’s responsibility to close the session when finished.
Example
This is very similar to the class method SMTP.start.
require 'net/smtp' smtp = Net::SMTP.new('smtp.mail.server', 25) smtp.start(helo_domain, account, password, authtype) do |smtp| smtp.send_message msgstr, 'from@example.com', ['dest@example.com'] end
The primary use of this method (as opposed to SMTP.start) is probably to set debugging (set_debug_output) or ESMTP (esmtp=), which must be done before the session is started.
Errors
If session has already been started, an IOError will be raised.
This method may raise:
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 459
def started?
@started
end
true if the SMTP session has been started.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 821
def starttls
getok('STARTTLS')
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 317
def starttls?
@starttls
end
Returns truth value if this object uses STARTTLS. If this object always uses STARTTLS, returns :always. If this object uses STARTTLS when the server support TLS, returns :auto.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 322
def starttls_always?
@starttls == :always
end
true if this object uses STARTTLS.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 327
def starttls_auto?
@starttls == :auto
end
true if this object uses STARTTLS when server advertises STARTTLS.
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 538
def tcp_socket(address, port)
TCPSocket.open address, port
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 287
def tls?
@tls
end
true if this object uses SMTP/TLS (SMTPS).
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 579
def tlsconnect(s)
verified = false
s = ssl_socket(s, @ssl_context)
logging "TLS connection started"
s.sync_close = true
ssl_socket_connect(s, @open_timeout)
if @ssl_context.verify_mode != OpenSSL::SSL::VERIFY_NONE
s.post_connection_check(@address)
end
verified = true
s
ensure
s.close unless verified
end
# File tmp/rubies/ruby-2.5.9/lib/net/smtp.rb, line 924
def validate_line(line)
# A bare CR or LF is not allowed in RFC5321.
if /[\r\n]/ =~ line
raise ArgumentError, "A line must not contain CR or LF"
end
end