I often need to send notification emails to myself from applications. It's not really reliable to authenticate your existing email account and send emails, as authentication can randomly be audited by robot checks and fail. Recently, I found how to send email with raw SMTP protocol without needing an account.

Assumptions & Requirements

  1. There's no firewall blocking you from contacting the recipient's mail exchange (MX) server port 25

    • For instance, at home I can't seem to contacting gmail MX server port 25.

  2. For Gmail at least, you can force your program to use IPv4 when communicating with the MX server, or you have a consistent PTR record if IPv6 must be used

  3. The emails would usually be delivered right into the spam folder. So you must be OK with it. You might create a rule to override it.

The workflow is that (inspired by this SO answer):

  1. Look up the MX server for the recipient email address by using nslookup mx. I find this online mx lookup service is invaluable.
  2. Talk with the MX server in SMTP to send the mail.

Here's an example SMTP session I had with gmail server. Lines marked with S> is server response, those marked with C> is from the client:

S> 220 mx.google.com ESMTP hj1si1000592pac.235 - gsmtp
C> HELO kfl.com
S> 250 mx.google.com at your service
C> MAIL FROM:<k@kfl.com>
S> 250 2.1.0 OK hj1si1000592pac.235 - gsmtp
C> RCPT TO:<kfl@gmail.com>
S> 250 2.1.5 OK hj1si1000592pac.235 - gsmtp
C> DATA
S> 354  Go ahead hj1si1000592pac.235 - gsmtp
C> From: "KL" <kfl@gmail.com>
C> To: "You" <kfl@gmail.com>
C> Subject: This is from me
C>
C> Hello there:)
C>
C> .
S> 250 2.0.0 OK 1462862782 hj1si1000592pac.235 - gsmtp

Note that if you're accessing internet through a regular consumer ISP, e.g., I'm using the crappy Comcast, firstly, you most likely is blocked from accessing port 25 of the MX server, I haven't tried SSL, maybe it will work. But from my work environment, I can access port 25 just fine.

Secondly, if you have IPv6 address and don't have a consistent PTR record (some sort of DNS reverse lookup strategy), you'll be blocked by Gmail at the end of the session, like this one. The workaround is to force the client to use IPv4. Linux telnet client has -4 command line option. Windows telnet client doesn't have that, but putty does have this option.

telnet.exe gmail-smtp-in.l.google.com 25

220 mx.google.com ESMTP m90si995173pfj.201 - gsmtp
HELO kfl.com
250 mx.google.com at your service
MAIL FROM:<k@kfl.com>
250 2.1.0 OK m90si995173pfj.201 - gsmtp
RCPT TO:<kfl@gmail.com>
250 2.1.5 OK m90si995173pfj.201 - gsmtp
DATA
354  Go ahead m90si995173pfj.201 - gsmtp
From: "KL" <kfl@gmail.com>
To: "You" <kfl@gmail.com>
Subject: This is from me

Hello there:)

.
550-5.7.1 [2001:4898:80e8::436] Our system has detected that this message does
550-5.7.1 not meet IPv6 sending guidelines regarding PTR records and
550-5.7.1 authentication. Please review
550-5.7.1  https://support.google.com/mail/?p=ipv6_authentication_error for more
550 5.7.1 information. m90si995173pfj.201 - gsmtp

Finally, don't forget to check the spam folder for the mail!