PocketPCSoft has had for some time a very usefull tool for POP3 accounts that have no anti-spam filters. This software is server independant which means you can centraly control your anti-spam filters localy on your PocketPC.
Here are some of the top features of this application:
Delete spam emails without downloading them
View and read your emails
Check all your email accounts simultaneously
Delete viruses and large attachments
Create powerful filters (example)
Filter by sender’s email or domain
Filter by sender’s country
Thousands of predefined filters and blacklisted domains
Automatically marks your Contacts as friends
Compatible with all PocketPC 2002, Windows Mobile 2003 / SE / WM5 devices
You can get more information and trial here.