How to add a Reply-To header to all e-mails sent from Mac OS X Mail

Recently I had to configure a POP3 e-mail account in Mail from an entity that doesn’t provide SMTP. In order to reply to people that sent messages to that account I had to configure Gmail as the sending mail server.
Later I found out that replies to messages sent from that account were sent to the Gmail account set as SMTP server.

Searching for an option in Mail to set the ‘Reply-To’ header, I found nothing. After some searching, I found the steps required to do so.

Note: Applying these steps will set the header for every message you send whatever the account you send it from. You also cannot replace the header with another one inserted in the ‘Reply-To’ message field while editing the message.

Here are the steps:

1. Open Terminal.
2. 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.
3. This will replace all contents of UserHeaders in com.apple.mail. You can check if UserHeaders is empty by typing “defaults read com.apple.mail UserHeaders” and then Enter.
4. If that command returns something other than “The domain/default pair of (com.apple.mail, UserHeaders) does not exist” you have to edit com.apple.mail.plist by typing “open Library/Preferences/com.apple.mail.plist” and then Enter.
5. Find UserHeaders in the editor and add the “Reply-To” with the address you want to use.
6. Save the file and you are done!

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