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?
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:
More details can be found here.
Just configure both email accounts with the same email address, but different server details and credentials.
You can set the e-mail address in Mail.app Preferences > Accounts for each account individually.