EmAn
How much memory do you have allocated to your mail program?
How much email do you have in the program, such as attachments that you are saving that bloat your memory requirements/needs?
Have you tried opening the mail program with only the required system extensions? You could have an extension conflict.
Check for other preferences, the most recent ones, and move them to a safe location away from the system folder (put them in a folder named, for example, TEMP MOVED PREFS, or something like that.) You might have other preferences that are corrupt, believe it or not, that might wreak havoc with your system.
Reboot and rebuild your desktop (command (apple) + option keys)
If all else fails you should zap your pram (where all your system settings/preferences are stored) three times. Let it restart three times and then see if your email program works. You will have to reset your customized desktop parameters, etc.
I hereby make a disclaimer indemnifying me from anything that might happen to your computer based on my suggestions. Zapping pram is a great reset, but has it's problems sometimes too.
Cheers!