My server is using Centos 6.5
I updated from Plesk 11.5 to 12 last week and postfix to 2.8.17.
Since then, all mails sent using the sendmail binary (notifications, mail forwards…) are being rejected with a wrong HELO hostname: localhost.
It seems that sendmail is using locahost as a HELO tag which is not accepted.

Qick Fix:

remove whole non_smtpd_milters from postfix main.cf in case you do not use commtouch.
If you would like to use commtouch then you can remove only “inet:127.0.0.1:12768” from non_smtpd_milters.
restart postfix.

Link: http://forum.sp.parallels.com/threads/postfix-mails-sent-through-sendmail-binary-are-blocked-because-of-wrong-helo.305216/