Messages arriving in batches

[tim.mothery]

Having a strange behaviour since upgrading to 6.08 from 6.05, where a user will be waiting for an email that they are expecting and it may not arrive for 20 - 30 minutes after it was sent (user is frequently clicking 'Get-Mail').

When it does finally show up in the web interface it will be all the emails from the last 20 -30 minutes. So the user will see, for example, a dozen email all arrive at a single click of 'Get-Mail' even though they have been clicking impatiently for the last half hour with nothing showing up.

I'm having a really hard time tracking this down since it is a frequent problem but doens't always happen. The emails once available in the web interface look perfectly normal -- including the date/times of arrival seem to be correct ie normally distributed over the last half hour and not all in the same minute or two.

Don't know what to do or how to further analyze what's going on. Any/all help is greatly appreciated.

Tim

6.08 community edition
CentOS 5.5 (totally stock except postgrey)
postgrey filter (has been there unchanged since zimbra version 4 and emails that are affected by this problem include those of frequent senders)