SMTP: Debugging SMTP with TLS/SSL and Auth

SMTP use TLS/SSL to secure connection to server and AUTH so only authenticated user can use the SMTP service. This tutorial will show steps to debug SMTP TLS/SSL and AUTH from Linux/Unix terminal.

####1. encode your login information in base64, the following perl command which requires MIME::Base64 will do encoding # perl -MMIME::Base64 -e ‘print encode_base64("\000your_username\000your_password”)’ AHlvdXJfdXNlcm5hbWUAeW91cl9wYXNzd29yZA== ####2. connect to smtp server normal non-secured SMTP # telnet smtp.yourdomain.com 25

TLS connection, check STARTTLS support with EHLO command
# telnet smtp.yourdomain.com 25
220 SMTP banner
EHLO smtp.yourdomain.com
250 SMTP banner
250-smtp.yourdomain.com
250-PIPELINING
250-SIZE 36360000
250-VRFY
250-ETRN
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN
quit
# openssl s_client -starttls smtp -crlf -connect smtp.yourdomain.com:25

SSL connection
# openssl s_client -crlf -connect smtp.yourdomain.com:465

####3. check AUTH support with EHLO command connect to secure SMTP using TLS or SSL # openssl s_client -starttls smtp -crlf -connect smtp.yourdomain.com:25 or # openssl s_client -crlf -connect smtp.yourdomain.com:465 … EHLO smtp.yourdomain.com 250-smtp.yourdomain.com 250-PIPELINING 250-SIZE 36360000 250-VRFY 250-ETRN 250-AUTH PLAIN LOGIN 250-AUTH=PLAIN LOGIN 250-ENHANCEDSTATUSCODES 250-8BITMIME 250 DSN quit ####4. use AUTH command to authenticate connect to secure SMTP using TLS or SSL # openssl s_client -starttls smtp -crlf -connect smtp.yourdomain.com:25 or # openssl s_client -crlf -connect smtp.yourdomain.com:465 … HELO smtp.yourdomain.com 250 smtp.yourdomain.com AUTH PLAIN AHlvdXJfdXNlcm5hbWUAeW91cl9wYXNzd29yZA== 235 2.7.0 Authentication successful if failed 535 5.7.8 Error: authentication failed: authentication failure ####5. test sending message connect to secure SMTP using TLS or SSL # openssl s_client -starttls smtp -crlf -connect smtp.yourdomain.com:25 or # openssl s_client -crlf -connect smtp.yourdomain.com:465 … HELO smtp.yourdomain.com 250 smtp.yourdomain.com AUTH PLAIN AHlvdXJfdXNlcm5hbWUAeW91cl9wYXNzd29yZA== 235 2.7.0 Authentication successful MAIL FROM: your_username@yourdomain.com 250 2.1.0 OK RCPT TO: your_destination@domain.com 250 2.1.5 OK DATA 354 End data with . From: Your Name your_username@yourdomain.com To: Your Destination Name your_destination@domain.com Subject: Your Email Subject Your Email Content . 250 2.0.0 Ok: queued as 6A4C1D5153E quit Connection closed by foreign host. Reference: https://qmail.jms1.net/test-auth.shtml