Sending messages from webmail fails: SMTP Error (250): Authentication failed

Symptoms

• Rouncube shows error:

  CONFIG_TEXT: SMTP Error (250): Authentication failed

• Checking server externally with telnet is not showing the AUTH option at all to the EHLO command:

  # telnet 203.0.113.2 25
  Trying 203.0.113.2...
  Connected to 203.0.113.2.
  Escape character is '^]'.
  220 203.0.113.2
  ESMTP Postfix
  EHLO example 
  250-203.0.113.2
  250-PIPELINING
  250-SIZE 51200000
  250-ETRN
  250-STARTTLS
  250-ENHANCEDSTATUSCODES
  250-8BITMIME
  250 DSN

• In the /var/log/plesk-roundcube/errors the following error may be shown:

  CONFIG_TEXT: SMTP Error: Authentication failure: SMTP server does not support authentication (Code: ) in /usr/share/psa-roundcube/program/lib/Roundcube/rcube.php on line 1667 (POST /?_task=mail&_unlock=loading1554712039855&_lang=en_US&_framed=1&_action=send)

Cause

Postfix misconfiguration.

Resolution

1. Connect to the server via SSH;

2. Check that smtpd_tls_protocols, smtpd_tls_ciphers and smtpd_tls_security_level, smtpd_sasl_auth_enable are set as below using the following command:

   # grep -E "smtpd_tls_protocols|smtpd_tls_security_level|smtpd_tls_ciphers|smtpd_use_tls|smtpd_sasl_auth_enable" /etc/postfix/main.cf
   smtpd_use_tls = yes
   smtpd_tls_security_level = may
   smtpd_tls_ciphers = medium
   smtpd_tls_protocols = TLSv1 TLSv1.1 TLSv1.2
   smtpd_sasl_auth_enable = yes

3. Make sure that smtpd_tls_exclude_ciphers, smtpd_sasl_security_options, smtpd_tls_auth_only and tls_ssl_options are not defined in /etc/postfix/main.cf file (the output of the command below should be empty):

   # grep -E "smtpd_tls_exclude_ciphers|smtpd_sasl_security_options|smtpd_tls_auth_only|tls_ssl_options" /etc/postfix/main.cf

4. Reload postfix if any changes were made:

   # service postfix reload