After a recent successful Server Migration from Exchange 2003 to Exchange 2010, I discovered (my phone was ringing off the hook) that there was a problem with users accessing their mailboxes via POP3. I later discovered that this problem only affected users that had been migrated and not users that were setup fresh on the Exchange 2010 server.
Having struggled to ascertain a reason why, I contacted Microsoft and they helped me to troubleshoot the problem.
After much time spent with Microsoft on the phone testing, tweaking, more testing and more tweaking, we were not getting anywhere.
Eventually they got me to download a capture program to monitor the problem and then we did some more testing and I sent them the captured info, which contained the errors in the POP3 collection process.
Anyway, to cut a relatively long and very unexciting story short, they advised me that there was a known problem with migrated users on Exchange 2010 who use POP3 to collect their mail. This problem only affects users who have been migrated (as I had discovered), so if you create a brand new email account and test POP3 mail collection, it will work fine.
The fix for this problem from Microsoft will be released in Exchange 2010 Rollup 2, so if you have not yet installed this Rollup (and it is available) and you are suffering from this particular problem, then please bring your server up-to-date.
At the time of writing this article (Feb 16th 2010), the Rollup has not been released, but it is rumoured to be appearing in late Feb / early March 2010.
As a workaround, I have exported all mail in the affected mailboxes, deleted the entire user account, recreated the account and put the mail back again. The user can then use their favourite email program to pull down the mail as expected.
Exchange 2010 Rollup 2 has been released now and you can download it from the following link: