Unable to send an email from Plesk server: SMTP Error (535): Authentication failed

Symptoms

• While trying to send an email via Roundcube, the following error appears:

  CONFIG_TEXT: SMTP Error (535): Authentication failed

• In /var/log/maillog following entries can be found:

  CONFIG_TEXT: warning: SASL authentication failure: cannot connect to saslauthd server: No such file or directory
  warning: SASL authentication failure: Password verification failed
  warning: localhost[::1]: SASL PLAIN authentication failed: generic failure

• saslauthd service is running:

  # service saslauthd status
  Redirecting to /bin/systemctl status saslauthd.service
  ● saslauthd.service - SASL authentication daemon.
  Loaded: loaded (/usr/lib/systemd/system/saslauthd.service; enabled; vendor preset: disabled)
  Active: active (running)

• compatibility_level value was previously defined in /etc/postfix/main.cf as following:

  CONFIG_TEXT: compatibility_level=2

Cause

Unclear.

Resolution

1. Login to the server via SSH.

2. Create a backup of /etc/postfix/main.cf:

   # cp -p /etc/postfix/main.cf /etc/postfix/main.cf_bck

3. Remove compatibility_level value from /etc/postfix/main.cf using the favorite text editor.

4. Check SMTP daemon configuration:

   • If RHEL/CentOS/CloudLinux is used, verify that chrooted mode is disabled:

     # egrep "^smtp " /etc/postfix/master.cf | grep inet
     smtp inet n - n - - smtpd

   • If Debian/Ubuntu is used, verify that chrooted mode is enabled:

     # egrep "^smtp " /etc/postfix/master.cf | grep inet
     smtp inet n - y - - smtpd

5. Reload postfix configuration:

   # service postfix reload