Over the past few months, I’ve noticed that an increasing number of replies to email that I’ve sent are “reactions”.

I imagine that, to the sender, or to someone in the Microsoft ecosystem, they are handled a bit liked a “thumbs-up” or “heart” reaction to a Signal message.

To me - as someone not in the Microsoft ecosystem - for each reaction, I get an email:

like [person] reacted to your message:

The “like” is alt-text. Because I don’t allow loading of remote content, I don’t see an image here.

I don’t want this.

Adding a header to postfix to attempt to stop it

Microsoft has a specific header which one can add to outgoing to email:

x-ms-reactions: disallow

My understanding is that, if that header is set, Microsoft suppresses the ability in its clients to respond with a reaction.

It is annoying that I need to set a specific header for this.

If every feature required a specific header to signal that it is unwanted, that would get irritating rather rapidly.

But at least there is this header.

If your mail client / MUA allows you to add headers, you could do it that way.

If you did, you wouldn’t need to tinker with your mailserver’s config.

But you’d need to do it for each client you use.

I want it to apply to all email I send, from whatever account, and from whatever device, so I added it to my postfix configuration.

In /etc/postfix/main.cf, I have a setting:

header_checks = pcre:/etc/postfix/header_checks

So I added my new header to /etc/postfix/header_checks.

I added

# add header to deal with unwanted Microsoft reactions (2024-07-16)
/^Date:/i PREPEND x-ms-reactions: disallow 

(Edit: originally, I put this line before the Content-Transfer-Encoding header. But my mutt configuration does send that header, so I’ve switched it to go before the Content-Type header instead, as all my MUAs send that. Further edit: I’ve put it before the Date header.)

and restarted postfix (sudo service postfix restart).

Then I tested it with various clients, and looked at the message headers to check that the header was being added correctly.

It was, so, success!

Now, will I get any more unwanted “reactions” email…? I’ll have to wait and see.

Update: Testing shows some success

I’ve tested this with a couple of people.

In one case, the header enrichment happened, but the Microsoft-using recipient still had the options to send a reaction.

They did, but the reaction did not reach me - it never hit my mailserver.

Microsoft sort-of foreshadows this when it says:

Since Disallow Reactions will roll out to different Outlook clients at different cadences and not all Outlook clients will have the gray-out update immediately, we also have a second layer of protection. When an email has reactions disallowed, attempts to react to it will fail at the server side.

My reading of this is that, even if a Microsoft client shows that reactions are available, and even if someone clicks it to send a reaction, Microsoft may still drop it silently.

That doesn’t feel like an ideal user experience (even if, for me, it achieves the goal of not getting a reaction email).

For the other Microsoft-using person, the reaction symbol was greyed out, and a hover text said “Reactions are disallowed on this message”.

Perhaps it does vary by Microsoft system, which again seems rather sub-optimal.