Hi Tarquin.
Verify/repair your disk (not just permissions), as described here:
The Repair functions of Disk Utility: what's it all about?
When an application crashes (i.e. quits unexpectedly), the system writes information about the state of the application at the moment of the crash into a log file. That information is called a crash report and it makes troubleshooting much easier. In order to find and paste the relevant part of the crash report here, proceed as follows:
1. Open
~/Library/Logs/CrashReporter/Mail.crash.log. The whole Mail crash log appears on a Console window (you can also open that file with any text editor). It may contain many crash reports. We only want the latest, which is located at the bottom of the file.
2. Look for a line that begins with "Date/Time:" as close to the bottom as possible, and another that says "Thread N Crashed:", where N is a number. Select everything from the "Date/Time:" line to the lines immediately following "Thread N Crashed:". Copy that text and paste it here.
Do not post the whole crash log, and do not include anything beyond the lines immediately following "Thread N Crashed:" either, just the relevant part.
Take a look at the following thread for examples of what a crash report looks like:
http://discussions.apple.com/thread.jspa?threadID=466701
any advice on how to back up my messages
Mail stores most preferences and all account settings in
~/Library/Preferences/com.apple.mail.plist. Everything else, including all your messages and mailboxes, is stored within the
~/Library/Mail/ folder.
Note: For those not familiarized with the ~/ notation, it refers to the user's home folder, i.e. ~/Library is the Library folder within the user's home folder.