Replaced a Dell PE2600 SBS2000 server with 3 new servers - one is running Exchange 2003.
User accounts and mailboxes were copied over from the SBS2000 server to the PDC and Exchange server.
The company has two offices connected via a VPN through two identical WatchGuard SOHO tc6.
Computers in first office where servers are located attach to the domain and use Outlook 2003 with Exchange accounts.
Computers in the second office are in a workgroup, and also use Outlook 2003 configured with Exchange accounts.
There are many laptops in the field running Windows XP Pro that use both Outlook 2003 configured with POP3 accounts and OWA. Same as before server replacement.
All appeared well - users configured with Exchange accounts were sending and receiving emails. At first it appeared that the POP3 users also were sending/receiving fine, but then all the POP3 users started to time out during receiving. At first, it appeared that the problem was random "corrupted" emails: a POP3 account for each user was setup (1 at a time) on an in-house computer and when it would stall while receiving, we would go into OWA and move the suspect email out of the Inbox and try again. Sometimes this would "correct" the problem, other times we would need to move additional emails out of the Inbox. Most of the "problem" emails were from outside senders, although there also were some from within the company. We worked late into the night, completing all the POP3 accounts. However, the next day the same timing-out problem started happening again to the POP3 users - again most of the "problem" emails originated outside the company.
What can be wrong?