Manual Testing of Mail with SMTP and TLS

If you're anything like me, you've used the telnet method to verify SMTP connectivity many times, over the years:

$ telnet smtp.mail.com
EHLO domain.com
MAIL FROM: alex@mydomain
RCPT TO: alex@someotherdomain
DATA
Subject: This is a test email
This is a test email.
.

In the modern era of TLS encrypted email connectivity, this no longer works.  However, there is a similar method, only requiring Perl or BASH, and OpenSSL.  Thanks to https://www.saotn.org/test-smtp-authentication-starttls/ for the details.

First, you need to create an authentication string that the remote mail server will accept.  With Perl, do this:

$ perl -MMIME::Base64 -e 'print encode_base64("\000username\@example.com\000password")'
AHVzZXJuYW1lQGV4YW1wbGUuY29tAHBhc3N3b3Jk

With BASH, do this:

$ echo -ne '\0username@example.com\0password' | base64
AHVzZXJuYW1lQGV4YW1wbGUuY29tAHBhc3N3b3Jk

Then, you can make your test connection to the mail server, using the handy "s_client" functionality of OpenSSL:

$ openssl s_client -connect smtp.gmail.com:587 -starttls smtp
CONNECTED(00000003)
<removed all the certificate junk>
---
250 SMTPUTF8
EHLO there
250-smtp.gmail.com at your service, [73.213.115.193]
250-SIZE 35882577
250-8BITMIME
250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8
AUTH PLAIN AHVzZXJuYW1lQGV4YW1wbGUuY29tAHBhc3N3b3Jk235 2.7.0 Accepted

See, it really wasn't very hard, was it?