I have Cingular, but I hear that all the providers are equally bad. One reason I got link2exchange was the fun I had convincing Cingular to get the wap.cingular apn both working and billed correctly. See
http://www.blackberryforums.com/showthread.php?t=7645
for the entire sad story.
I use squirrelmail for my webmail interface. It's terrible as a reader, but great as a configuration tool. In squirrelmail 1.4.4 I can login to my account, and under options is a new line "server side message filters". These things act before you see your mail, using ANY reader from anywhere. Clink on that, add a new rule, and one of the options is
Redirect to the following email address:
which you can set to anything. An option below that is "keep mail on server".
We just got this version of squirrelmail. You may not have it where you are.
I'm amazed at how good this is.