SymptomsIt is Parallels Plesk Panel version 9 installed in Virtuozzo Container.
Message Transfer Agents (MTA) Postfix takes too much resources. privvmpages and numothersock reached their limits, and so mail service works unstable.
ResolutionBy default Postfix has big maximum number(100) of parent SMTP processes: delivering, filtering and other setting-up actions. Mail hangs and resources reach their limits if outgoing email messages cannot be delivered due to one of setting-up action above.
Use environment variable postconf to decrease maximum number of parent SMTP processes.
~# postconf -e 'default_process_limit=20'
Also it is recommended to disable unused services like ifmail and uucp in Postfix configuration /etc/postfix/master.cf (/usr/local/etc/postfix/master.cf for FreeBSD).
And reload MTA configuration with mailmng:
~# PRODUCT_ROOT_D/admin/sbin/mailmng --reload-service
Reloading postfix: [ OK ]
Additional informationUse the following article to get value of PRODUCT_ROOT_D for your system: