Symptom
Mail unexpectedly quits when sending a message.
This happens when one or more third-party mail bundles are installed.
Solution
Remove any third-party mail bundles installed when the issue began, or remove all bundles. These bundles are usually found in this folder:
Contact the publisher of any bundles that do not seem to work with Mail for more information--a later, more compatible version may be available.
Additional information
Examples of third-party mail bundles include those that provide encryption (PGP), message icons, or headers that may be prioritized.