Apple Mail 4.5, OS X 10.6

How to delete and re-add your Exchange mail profile

To delete your profile, select Preferences from the Mail drop down menu.

Select your Exchange account under "Accounts" and click the "-" button.

When prompted, confirm the removal of your Exchange profile by selecting "Remove".
When complete, quit Apple Mail and re-open it.

Once Apple Mail has re-opened, you will be asked for your Exchange information.
Enter your information and select "Continue."

On the Account Summary window select "Create".

Apple Mail 5 & 6, OS X 10.7 & 10.8

How to delete and re-add your Exchange mail profile.

To delete your profile, select Preferences from the Mail drop down menu.

Select your Exchange account under "Accounts" and click the "-" button.

When prompted, confirm the removal of your Exchange profile by selecting "Remove".
When complete, quit Apple Mail and re-open it.

Once Apple Mail has re-opened, you will be asked for your Exchange information.
Enter your information and select "Continue."

On the Account Summary window select "Create".