Any Exchange 2003 experts?

I'm stuck an a wild one, can't find the answer.

Customer wants to POP3 (from home, therefore remotely) to the Exchange server at the office. So we opened port 110 on the firewall. MS Exchange POP is enabled and started in services.

We decided that he would test it first internally, in the office (Small network) but it gives him the following error:

"the pop3 sever refused to allow a connection, check your mailbox user name and password. Also check the account is not being used elsewhere."

Needless to say, this MS error is not an error that helps diagnosing, and it comes from whatever workstation on the LAN, all of which are different. Normally the LAN runs SMTP email using the normal classic lookOut/Echange combo

The network has a 3.0Ghz Xeon Win 2003 server running the Exchange 2003 ... on mirrored disks, Raid 1 I think, 1 Gb RAM etc. There is another server whose only job is to store data.

Any suggestions would be more than welcome, I'll do my best to answer any questions, so long as they are guided towards an Exchange noob please!!

Ta muchly
N.