I have has this issue a good couple of months now and it is proving difficult to get to the bottom of..
Exchange 2003 SP2 FE
Exchange 2003 SP2 BE x 4
ActiveSync works on ALL exchange servers however there seems to be an issue on all but one of the servers where devices will fall out of sync. We have tested this to exhaustion and have come up with the following trends
New mailboxes sync ok and continue to sync as long as a particular sync doesn't contain more that 30,000 chars, this can be one email or multiple emails. Removing the suspect email(s) from the mailbox via outlook means the sync works again until more large emails are received
Existing mailboxes sync depending on the contents of a mailbox, eg my mailbox will only sync folders and no content, if I delete all emails then sync works until a large email is received.
This pattern is the same across all BE servers expect one, on this working server ExchBE01 ActiveSync works regardless of the mailbox content
If I run an exchange connectivity test on a mailbox on either ExchBE02-04 which contains large emails all tests pass expect the item sync which times out..
There is currently no desire on this platform to upgrade exchange OR add an alternative solution (as much as I would love to!)