> Dont know How can I check
Click your Calendar folder, switch to a category view so that all items are
available, and examine the number that appears at the bottom left in the
Status bar.
> and why does that matter?
Folders have a limit on the number of items they can hold. It's pretty
high, though.
> It keeps giving me the "messaging interface has returned and unknow error"
If this were happening to me, I'd do the following, in this order:
1) Create a new PST (if I were using a PST for my delivery location), make
it my delivery location, then copy all the data in my old PST to the new
one. If I were using Exchange as my delivery location, I'd complain to the
Exchange admins.
2) Create a new mail profile, pointing it at my existing data store.
3) Run Detect and Repair
4) Uninstall and reinstall Outlook
5) Perform 1 and 2 over if needed.
6) Pay Microsoft for some consulting.

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Brian Tillman