How to Add a Reply-to Header in Emails (Mac OS X Mail)

Step-by-Step Instructions For Adding a Reply-To Header

You know how to manually add a "Reply-To" header to an email you are composing in Mac OS X Mail, and you do find it useful. But this can also be cumbersome if you want every outgoing email to carry that information automatically—and that's what you want.

Fortunately, Mac OS X Mail has a somewhat concealed facility that lets you add any header—include a "Reply-To" line—to emails you send automatically. It's not exactly straightforward, but here are step-by-step instructions.

Add a Reply-to Header to All Emails You Send in Mac OS X Mail

To make Mac OS X Mail add a custom "Reply-To" header line to all emails you send:

  • Open Terminal.
  • Type "defaults read UserHeaders".
  • Press Enter.
  • If that command returns "The domain/default pair of (, UserHeaders) does not exist":
    • Type "defaults write UserHeaders '{"Reply-To" = "reply-to@address"; }'" (excluding the outermost quotation marks) and replace reply-to@address with the address you want to be used for replies.
  • If the "defaults read" command above returns a line of values that starts with "{" and ends in "}":
    • Highlight the entire line. It might read something like {Bcc = "bcc@address"; }, for example.
    • Press Command-C.
    • Type "defaults write UserHeaders '" (excluding the outer quotation marks).
    • Press Command-V.
    • Type "'".
    • Insert '"Reply-To" = "reply-to@address"; ' in front of the closing "}" character, replacing reply-to@address with the email address you want to be used for replies.
      • The line might now read defaults write UserHeaders '{Bcc = "bcc@address"; "Reply-To" = "reply-to@address"; }', for example.
  • Press Enter.

How to Change Your Reply-To Header

Unfortunately, this method of adding a Reply-To header works too well in some situations. Once in place, there is no way to make Mail not append it to a particular mail. You cannot change the address while composing a message either.

To make any changes to the Reply-To header, you have to go the route of the terminal. Again, it's not exactly straightforward, but here are step-by-step instructions on how to change the address in the Reply-To Header.

How to Disable Automatic Reply-to Headers in Mac OS X Mail

To turn off all "Reply-To:" headers:

  • Execute "defaults delete UserHeaders" at a Terminal prompt to delete all custom headers or set UserHeaders back to what it was before you added "Reply-To".