Is there any way to run a script whenever the user tries to send mail? I'm particularly interested in ensuring that outbound mail doesn't have a blank subject line.
Solutions that involve plug-ins are welcome!
It looks like there's no easy way to do this via AppleScript, however there's a plugin that warns in case of blank subject, it's Subjectivity. It works only with Tiger, since Leopard and Snow Leopard's Mail.app warns you if you leave subject blank.
I found this blog post on writing Mail plugins, however I didn't try to write one.