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.
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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")' # example output # AHlvdXJfdXNlcm5hbWUAeW91cl9wYXNzd29yZA== -
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 -
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 -
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 -
test sending message
$ 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 <CR><LF>.<CR><LF> 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>