Mail.app problems

[TheMacintoshLady]

What can I do to stop Mail. app from constant crashes and quits.

Also, have one address that seems to be a  problem.

Every time I open Mail.app, even if I'm not looking in that particular 
mailbox I get a weird message stating that this mailbox has been locked 
by me and it will damage if opened.

Checked permissions - same on all boxes, none others have problems
Ran permissions which seemed to fix for a while but then started again.
Temp solution is I deleted that address/related boxes and will wait to 
see what is wrong before I try to add it in again.

Not sure what is wrong, other addies do not do that.
Thanks....