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Use the Reply-To Header in Emails

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I should have written this article long time ago because according to my experience, most people don't know this very useful functionality of the email standard.

Most of you might know about the email headers Cc: and Bcc:. Only few know the email header Reply-To: which is as great as the other headers mentioned. And this is explaining why you should know about it and use it when appropriate.

Example Scenario

Consider yourself writing an email to a group of people and you ask them to send answers to a different email address than your sender address. This might look like the following email example where Susan sends out a request to her friends:

From: Susan <susan@example.com>
To: bart@example.com, lisa@example.com, homer@example.com
Subject: Garden Party on 2021-07-31: who brings what salad?

Hi,

Who will bring what salad to the party next week?

Please send your salad plans to Peter until Friday.

Thank you and have a great day	  

I guess that most of you will agree that emails like this are a common pattern.

There is an obvious issue: everybody who answers the email of Susan needs to manually look up Peter's email address and replace Susan's address in the reply before sending their salad plans.

This results in two potential problems. First, people who don't care or who are in a stress situation, not reading thoroughly might send their answer back to Susan instead to Peter who is waiting for their salad decisions. Second, people need to have Peter's email address in order to be able to comply with Susan's request.

Both problems can be solved in a very clever way by using a standard email feature which is around for a couple of decades but which is also a less-known one. I would like to change this and explain its usefulness to you and how you might apply it into your daily email routine.

The Reply-To Email Header

The Reply-To: email header is a header just like Subject:, From: or Cc:. Unfortunately, most email clients do only show a few email header elements when composing an email and hide the majority. Most of the mail clients do allow the use of Bcc: or Reply-To: headers in some form. Some require you to enable them in the settings so that they are displayed along the other standard headers, some require you to go to some sub-menus in order to manipulate its value for each email just like Outlook does.

My current mail client enables the display of Bcc:/Cc:/Reply-To: in a menu of the email compose dialog.

Okay, now we know how to access this header element. But what for?

Remember the garden party example from above? With the Reply-To: header, we are able to improve the garden party email in this way:

From: Susan <susan@example.com>
To: bart@example.com, lisa@example.com, homer@example.com
Reply-To: Peter <peter@example.com>
Subject: Garden Party on 2021-07-31: who brings what salad?

Hi,

Who will bring what salad to the party next week?

Please send your salad plans to Peter until Friday by simply replying
to this email.

Thank you and have a great day	  

Notice the additional Reply-To: header line with Peter's email address? When any receiving person replies to Susan's email, their mail client pre-fills the To: field with Peter's email address from the Reply-To: header entry instead of Susan's email address.

Of course, you are still able to change the recipient of the email manually to a different one.

Some email clients issue a warning message when Reply-To: is set because attackers might use this feature to trick people to send sensitive information to an externally controlled mail address. That warning is good and in most cases it should be fully OK to respect the different recipient address after checking it carefully which is more difficult in some insecure mail clients like Outlook.

Interesting to know that you might write multiple email addresses into the Reply-To: field:

From: Susan <susan@example.com>
Cc: Peter <peter@example.com>
To: bart@example.com, lisa@example.com, homer@example.com
Reply-To: Peter <peter@example.com>, Susan <susan@example.com>
Subject: Garden Party on 2021-07-31: who brings what salad?

Hi,

Who will bring what salad to the party next week?

Please send your salad plans to Peter until Friday by simply replying
to this email.

Thank you and have a great day	  

In this example, both organizers (Susan and Peter) gets the replies by default.

Using this technique is also handy when functional email addresses such as office@example.com or support@example.com are involved:

From: Susan <susan@example.com>
To: Bob.Client@customer.example.com
Reply-To: office@example.com
Subject: Re: What are your opening hours?

Hello Bob,

Bob Client wrote:
> Hi, what are your opening hours in the COVID sitation?

We do have the same opening hours as before.

If you do have any further questions, please do reply to this email.

Thank you and have a great day	  

As a matter of fact, you can even think of having multiple email addresses in the From: header (which would also be used as recipient addresses for standard replies in case there is no Reply-To: header set):

From: Susan <susan@example.com>, office@example.com
To: Bob.Client@customer.example.com
Reply-To: office@example.com
Subject: Re: What are your opening hours?

Hello Bob,

Bob Client wrote:
> Hi, what are your opening hours in the COVID sitation?

We do have the same opening hours as before.

If you do have any further questions, please do reply to this email.

Thank you and have a great day	  

However, when you do use multiple sending addresses, this does not send any email to the office address (in this example) as long as you don't include the office address in one of To:/Cc:/Bcc:.

If you're using email lists such as customers@example.com to send out emails to a larger audience, you also might want to make sure that a Reply-To: header is set to a different email than the email list. Otherwise, replies would be sent to the whole list by default which is - quite unfortunately - a common habit.

I hope I could teach you a useful trick for your daily email routine. Please do forward this URL to your friends and colleagues to that you might profit from correctly set Reply-To: headers as well.

Comment: Alexandros

Hi. Very nice article.
I wasnt aware either.. 25 years using email .. go figure..
Anyway congratulations for the article.

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