I have recently had a number of run-in's with the SBS 2003 POP3 connector and the thing's it DOESN'T do properly, such as not duplicate mail incessantly or just plain drop mail addressed to BCC - blind carbon copy recipients.
With that in mind, I've looked at the options for a POP connector replacement which essentially boils down to
a) connecting to a catch-all or list of defined mailboxes
There are a vast number of options to choose from. A number of commercial solutions can be found on Msexchange.org, freeware or open source solutions are available as well, and are very well implemented. One of the largest pains that follows POP based mail is the lack of native SPAM filtering, dude to the POP connector bypassing IMF altogether. The POP connector uses CDO to deliver mail as opposed to submitting it to the SMTP stack for pickup, classification and routing.
I have found a really good, solid replacement for the POP connector in the form of Chimera Computing's free (you heard right) pop connector. you can find them here: http://www.chimera.co.nz/ . Their connector is functional and cheap if you need to license it - advanced functions need activation, however to troubleshoot POP their free connector is awesome!
When it comes to a POP connector with LOADS of ANTI spam features - checkout GFI. They're a bit light on pure POP functionality, however they're quite feature rich in terms of SPAM and Exchange 2003 anti SPAM feature integration.