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


A Reply-To Header Not Once, Not Twice, But Always

You know how to add a "Reply-To" header to an email you are composing in Mac OS X Mail, and you do find it useful. But also cumbersome if one wants every outgoing email to carry that information—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.

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 com.apple.mail UserHeaders".
  • Press Enter.
  • If that command returns "The domain/default pair of (com.apple.mail, UserHeaders) does not exist":
    • Type "defaults write com.apple.mail 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 com.apple.mail 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 com.apple.mail UserHeaders '{Bcc = "bcc@address"; "Reply-To" = "reply-to@address"; }', for example.
  • Press Enter.


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.

Disable Automatic Reply-To Headers Again in Mac OS X Mail

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

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

(Updated September 2011)

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