After setting the default "Message font" in Mail Preferences to Verdana, it reverts, on its own, to Helvetica. I suppose I can understand why, if someone sends me an email set in Helvetica, it remains in Helvetica for my reply. What I don't understand is why the default Message font in Mail Preferences would ever change on its own, which is what it's doing.
Sounds like a corrupted preferences file.
Go to /Library/Preferences (in your startup hard drive) and find a file called com.apple.mail.plist, and drag it to the trash. Restart your computer, or log out of and then back into your user account. This will generate a brand new plist file with default settings, so you will have to enter your preferences again, but this time they'll stay put.
Do you mean the email you sent doesn't arrive in the font you composed it in, or when you compose a new message, it changes.
The first can happen if the receiving email client doesn't process the email correctly. You can often "force" it by changing the font while composing.
For the latter, I would concur with Arthur, but with the sandboxing and the new Mail data structure, I'm not sure which plist to delete.
Default "Message font" in Mail changes to Helvetica
