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:

  SMTP Error (535): Authentication failed

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

  compatibility_level=2

Cause

The root cause is under investigation.

Resolution

1. Login to the server via SSH.

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

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

3. 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

4. Reload postfix configuration:

   # service postfix reload