This is a really easy fix to permanently add a reply-to to Apple's mail.app. Open a terminal and adjust the following by inserting the email address that you want recipients to reply to.
defaults write com.apple.mail UserHeaders '{"Reply-To"="tracy.phillips@example.com";}'
If you later decide that is not for you, then you can get back to the default by issuing
defaults delete com.apple.mail UserHeaders
Tracy — thanks for the terminal command. Question — is there any way to add a parameter to that command that only adds the populated Reply-To field when sending from a certain address?
I've got a few accounts set up under mail.app, and I don't want them ALL to be replied to the same address....