Delays in mail deliveries

[Damian]

>> does not match $max_servers.
> You mean I should change the processes number below (2) to match 
> amavis $max_servers configured value (currently 10)?

This would be one possible way, yes. A good number depends on cpu and 
memory usage and availability. SpamAssassin can be greedy, and clamscan 
(not clamdscan) as av_scanner will be even worse.

See also README.postfix chapter 5.1.

>> These are the wrong options for an lmtp service.
> Do you suggest to remove them entirely or change their values?
I suggest to use the lmtp_* counterparts.
>> Beyond that, delays of 120s and 300s look suspiciously man-made. I 
>> don't have a concrete idea, though. This may be a question for 
>> postfix-users (together with related logs from vmail2).
>
> Could a possible cause be that postfix default_process_limit is 
> currently as low as 10?
I don't know.
>
> Would it possibly help to also increase amavis $max_servers to a 
> higher value?
For your original problem? I doubt it.