Symptoms

After upgrade to Ubuntu 16.04 the following messages start appearing in mail queue:

# mailq
-Queue ID-  --Size-- ----Arrival Time---- -Sender/Recipient-------
90EC33500755     2804 Thu Nov  3 16:10:24  MAILER-DAEMON
(Command died with signal 6: "/usr/lib/plesk-9.0/postfix-local". Command output: *** stack     smashing detected ***: /usr/lib/plesk-9.0/postfix-local terminated)
                                     test@example.tld

Cause

This is bug with ID PPPM-5165 which is planned to be fixed in future Plesk updates. Mail SRS library could not change from into SRS format.

Workaround

Add the following parameter into the /etc/postfix/main.cf :

empty_address_recipient = MAILER-DAEMON@localhost.localdomain