1. Checking the size of personal folder and user's mailbox, make sure it won't over 2.0 GB, otherwise, you must strongly recommend user to create a new personal folder. Afterwards, please go to "Rules and Alerts" to disable/modify the old rule if it redirect the incoming mails to old personal folder.
2. Checking the rules of outlook, make sure there's no conflict, you may check it one by one and possible to uncheck all while you are in a trouble that related on rules.
3. Checking the Advanced Options for Add-in and COM Add-ins, to remove the non-standard components.
4. Backup the current outlook profile settings,
a. Export Rules
b. Signature
c. Outlook Special Settings, (Reg exporting)
d. Profile Backup
5. Uncheck the Cached Exchange Mode and Disable "Offline use" to see if problem still exists.
6. Make sure all the Regional settings are standard, and for Microsoft Office 2003 Language settings as well.
7. Creating a new profile for Outlook but don't remove the current one.
8. Restarting the PC, try to see if problem still exists.
9. Create a new outlook profile with your own account and then to see if problem still exists, in case of, problem still exists -- we know it relates on unknown system problem, going to check.
10. Logoff and re-logon with your domain ID and create a new outlook profile for user. (Should got the password from user), if problem still exists -- we know it relates on unknown system problem, going to check. If problem gone -- recreate a windows profile for user.
11. For user who cannot send mail out, you may check the delivery receipt after checking the size of mailbox
12. Repair corrupted Personal folder (Pst file), Please must backup the pst file before actions, its location is: "C:\Program Files\Common Files\System\MSMAPI\1033\scanpst.exe" and sometimes you can use "EasyRecovery Professional" or other tools for recovering.