Please provide the following information:
1) Version of Outlook - you may disregard this one
2) Email environment: Exchange, POP3, etc.
3) Does you have a PST? If so, what is the size?
4) Are there any error messages/numbers?
5) Have you checked the application event viewer for any Outlook or
Office-related errors? If there are, what are the Event ID number(s) &
source(s)?
6) What troubleshooting steps have you taken yourself, so you're not told to
take steps you've already tried?
7) Is the antivirus scanning incoming/outgoing mail?
8) What is the polling frequency for mail?
9) Are there any COMs, Add Ins, synch software (for handheld), Fax software
running?
10) Have you checked the configuration of your account in the Control Panel?
11) What changed between the last time it worked and the time it stopped
working?