I just installed Mountain Lion after using Lion and had all SORTS of problems with Mail application (freezing, deleted emails would reappear, crashing, etc. etc.). I called Apple Support and they told me to do this, and it works:
shut down
then turn on computer while holding command R
when OS Utilities window comes up go to the top menu bar and click Utilities, then Terminal.
type (all one word, lower case): reset password
hit return
go to the bottom of the reset password window that comes up,
select the HD
then click the Reset button at the bottom under "Reset Home Folder Permission and ACLs"
then wait for it to finish
click the red button to close the finder.
then go to Terminal at top,
Quit Terminal.
Then restart under the apple logo icon.
...This resets the Permissions for all the applications in your computer and refreshes a lot of things that cause bugs, they say....