How to set reply-to address in Apple Mail I have two accounts set up in Apple Mail. The second account is for backup purposes and because at times the primary account is unavailable. When that happens, I send from the second account but would like the reply-to address to be that of the primary account. Is there a way to set things up this way in Apple Mail?
Simple!

With a message composition window open in Apple Mail make the following menu selection:

View > Reply-To Address Field

That works for a message-by-message basis. If you want to apply all the time there's a hack you can do this via Terminal:

defaults write com.apple.mail UserHeaders '{"Reply-To" = "reply-to@address"; }'


More details can be found here.