On a really busy Postfix server you might encounter the following issue:
Jan 18 16:41:21 mail postfix/qmgr[1669]: warning: you may need to increase the main.cf local_destination_concurrency_limit from 2
A full transcript is at the end of this article.
Essentially this means Postfix is working too hard. This could be the case if many messages are timing out, either due to a spammer sending too much email to unknown addresses, or some other kind of breakdown (e.g. DNS issue).
Tail the log file in more detail to find the cause.
If you want to give your Postfix server some breathing room, it’s pretty obvious what to do.
Full transcript on a live server where this problem happened due to a known spammer:
Jan 18 16:36:21 mail postfix/qmgr[1669]: warning: mail for mail.example.com is using up 14833 of 14914 active queue entries Jan 18 16:36:21 mail postfix/qmgr[1669]: warning: you may need to increase the main.cf local_destination_concurrency_limit from 2 Jan 18 16:36:21 mail postfix/qmgr[1669]: warning: please avoid flushing the whole queue when you have Jan 18 16:36:21 mail postfix/qmgr[1669]: warning: lots of deferred mail, that is bad for performance Jan 18 16:36:21 mail postfix/qmgr[1669]: warning: to turn off these warnings specify: qmgr_clog_warn_time = 0