you may need to increase the main.cf ... destination_concurrency_limit from 20

[Donovan Bray]

I am seeing the following in the postfix logs, and we are having a delivery problem to blarg.com. However the destination_concurrency_limit reported at 20 took my by surprise. I was under the impression we had already set it higher.

Feb 11 12:36:08 email postfix/qmgr[23132]: warning: mail for blarg.com is using up 4001 of 4812 active queue entries

Feb 11 12:36:08 email postfix/qmgr[23132]: warning: this may slow down other mail deliveries

Feb 11 12:36:08 email postfix/qmgr[23132]: warning: you may need to increase the main.cf  smtp104_120_110_27_destination_concurrency_limit from 20

Feb 11 12:36:08 email postfix/qmgr[23132]: warning: please avoid flushing the whole queue when you have

Feb 11 12:36:08 email postfix/qmgr[23132]: warning: lots of deferred mail, that is bad for performance

Feb 11 12:36:08 email postfix/qmgr[23132]: warning: to turn off these warnings specify: qmgr_clog_warn_time = 0

We use sender dependent transport maps to allow our applications to send out the correctly plumbed sending-ip.

from master.cf

smtp      inet  n       -       n       -       -       smtpd

smtp104_120_110_27 unix  - - n - 150 smtp -o smtp_bind_address=104.120.110.27 -o syslog_name=postfix-app1 -o smtp_helo_name=mail-27.redacted.com

root@email:/var/log# postconf -n | grep destination_concurrency_limit

smtp_destination_concurrency_limit = 75

root@email:/var/log# postconf -d | grep destination_concurrency_limit

default_destination_concurrency_limit = 20

lmtp_destination_concurrency_limit = $default_destination_concurrency_limit

local_destination_concurrency_limit = 2

relay_destination_concurrency_limit = $default_destination_concurrency_limit

smtp_destination_concurrency_limit = $default_destination_concurrency_limit

virtual_destination_concurrency_limit = $default_destination_concurrency_limit

If I am understanding the implications correctly the currently set 'smtp_destination_concurrency_limit = 75' ONLY controls the limit for the smtp line in master.cf; it's not an inherited default for any 'smtp' service like we had assumed.

If my intention is to increase all of the smtpYYY_YYY_YYY_YYY interfaces then I need to set 'default_destination_concurrency_limit = 20' higher. (and 75 is probably way too high) 

OR create entries for the specific transports in main.cf like

smtp104_120_110_27_destination_concurrency_limit = 30

I'm assuming that over time we set it higher, and not seeing a negative effect kept increasing it, but there is almost no mail that goes out the default smtp interface, so we've been effectively running at a destination_concurrency_limit of 20 this entire time on all of our custom interfaces. 

I also understand raising the destination_concurrency_limit isn't going to solve the deliverability problem; it will only correct our understanding and intent regarding the configuration of our default_destination_concurrency_limit for this and other interfaces.

Is my resulting understanding correct?