Error in plesk postfix email logs:

Notice: Error: EHLO not accepted from server! in /var/www/vhosts/domain.com/httpdocs/system/library/mail.php on line 237

Check postfix main and comment out non_smtpd_milters:

smtpd_milters = , inet:127.0.0.1:12768
#non_smtpd_milters = , inet:127.0.0.1:12768

Restart postfix:

# service postfix restart
Shutting down postfix:                                     [  OK  ]
Starting postfix:                                          [  OK  ]