In a stroke of genius, Microsoft built a 2 Gig limit into Outlook .PST files, but allow this to be exceeded. This means you can find yourself unable to access your email folders and unable to delete files to get it back down below 2 Gig.

What to do if you pass the limit?
You can either pay top dollar for a program to sort this out for you, or use PST2GB which is free. Be aware though that this program will simply delete a chunk of your PST to get it below the limit again. If this causes any corruption to your email, the Inbox repair tool may be needed (see below).

Related: Microsoft Inbox Repair Tool Info