Are you experiencing POP3 issues with Outlook 2007?  Here is a couple articles you should check out.

http://support.microsoft.com/default.aspx?scid=kb;EN-US;935400

An Outlook update was released that provided significant performance benefits, especially for users with large stores (the download can be found at http://www.microsoft.com/downloads/details.aspx?FamilyID=c262bcfd-1e09-49b6-9003-c4c47539df66&DisplayLang=en).