Unable to send an email from a Plesk server with Postfix: SASL login authentication failed

Symptoms

• When sending an email from webmail, the following error message is shown:

  PLESK_INFO: SMTP Error (535): Authentication failed.

• The following error message appears in /var/log/maillog:

  postfix/smtpd[7538]: warning: unknown[203.0.113.2]: SASL login authentication failed: authentication failure

• The saslauthd service is up and running on the server:

  # service saslauthd status
  …
  Active: active (running) since …

Cause

The Postfix smtpd.conf configuration file is corrupted.

Resolution

1. Connect to the Plesk server via SSH.

2. Open the smtpd.conf file in a text editor (in this example, we are using the vi editor) and edit the file as follows:

   • on CentOS/RHEL-based distributions

     # vi /usr/lib64/sasl2/smtpd.conf

     pwcheck_method: auxprop saslauthd
     auxprop_plugin: plesk
     saslauthd_path: /var/spool/postfix/private/plesk_saslauthd
     mech_list: DIGEST-MD5 CRAM-MD5 PLAIN LOGIN
     sql_engine: intentionally disabled
     log_level: 4

   • on Debian/Ubuntu-based distributions

     # cat /etc/postfix/sasl/smtpd.conf

     pwcheck_method: auxprop saslauthd
     auxprop_plugin: plesk
     saslauthd_path: /private/plesk_saslauthd
     mech_list: DIGEST-MD5 CRAM-MD5 PLAIN LOGIN
     sql_engine: intentionally disabled
     log_level: 4

3. Save the changes and close the file.

4. Restart the saslauthd service:

   # service saslauthd start