I’ve had perpetual problems with greylisting. Not only do emails arrive late, many don’t arrive at all thanks to mailservers that don’t retry correctly. I’ve noticed many mailservers will retry almost immediately after the initial failure, several times in a row, before simply giving up.
Rather than disabling greylisting altogether, I’ve set the delay very low, to 1 second. Since most spammers don’t bother retrying at all, 1 second is enough to get rid of most spam, but is enough for legitimate mailservers which are simply not configured correctly.
To reduce the delay to 1 second, modify /etc/default/postgrey and change the postgrey options to read as follows:
POSTGREY_OPTS="--inet=127.0.0.1:10023 --delay=1"
You’ll need to restart postgrey after making this change:
service postgrey restart
Note that this is considered a hack, and may be overwritten by future updates to Mailinabox.