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[greylisting] mails are much longer rejected than configured

Lars

New Pleskian
(Panel-Version 11.5.30 Update #51 on Ubuntu 12.04.5 LTS)

We encounter the situation that the 'grey_listing' plugin sometimes reject a mails much longer than expected.
Last time this happens the mail was locally accepted over thirty hours later. The log file is not showing anything unusual:

Jan 7 11:11:11 server greylisting filter[11359]: Starting greylisting filter...
Jan 7 11:11:11 server /usr/lib/plesk-9.0/psa-pc-remote[646]: handlers_stderr: DEFER
Jan 7 11:11:11 server /usr/lib/plesk-9.0/psa-pc-remote[646]: DEFER during call 'grey' handler
Jan 7 11:11:11 server /usr/lib/plesk-9.0/psa-pc-remote[646]: Message aborted.

Going through the logs for this mentioned last case I figured out that the sender was using Microsoft's Mail Service "outlook.com". This service has a large amount of servers (IPs) to send remote mails. In our case it was always a different IP for every single (rejected) delivery attempt. And then thirty hours after the first attempt, as an "known" Remote-IP was trying again, the mail was locally accepted.

I don't know what parameters 'grey_listing' is using to identify delivery attempts but the remote IP might be not such a good idea? Any idea what to do to avoid those massive delays?
 
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