The Mail utility uses a lock file to prevent two or more processes from altering your mail spool file at the same time. If Mail Tool quits unexpectedly, the lock file may be left behind.
If Mail Tool takes a long time to open the In-Box, either when you specifically request it or when you open Mail Tool after you have chosen Done from the File menu, check for a lock file. Look in the directory /var/mail for a file named username.lock, where username is your login name.
To remove the file, from a system prompt type:
For example, if your username is "mary", you would type:
Alternatively, you can find the lock file in the File Manager, and drag it to the Wastebasket to delete it.