Adding custom header on outgoing mail

Neil

• July 17, 2024 08:46

I am attempting to stop Microsoft users sending 'reactions' to email from our server by adding a custom header. The header that needs to be added is:

x-ms-reactions: disallow

MS docs: https://techcommunity.microsoft.com/t5/outlook-blog/reactions-in-outlook-public-usability-update-september-2023/ba-p/3928103

I can't find any documentation on this for cPanel. Is this possible to at both a server, user, or domain level?

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  cPRex Jurassic Moderator

  • July 17, 2024 18:18

  Hey there!  Yes, Exim, the mail transport that cPanel uses, does support adding custom headers.  However, *we* don't support doing that work or troubleshooting that for clients, as then customizing email would likely turn into our entire job.

  I did some research on this and couldn't find anyone who has implemented this in Exim yet, so I'm wondering if this option is specific to Microsoft systems.

  Unfortunately I just don't have a good answer on this one and you may need to look into custom Exim headers to find out where and if this can be implemented.

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  Neil

  • July 18, 2024 10:15
  • Edited

  cPRex Thanks for your thoughts and, yes, I understand that this would be out of scope for cPanel support as it would properly open a can of worms!

  I got the original idea from https://neilzone.co.uk/2024/07/attempting-to-stop-microsoft-users-sending-reactions-to-email-from-me-by-adding-a-postfix-header/

  I think what I was looking at was that, at a high level, whether this approach would work in a cPanel environment and that it wouldn't dramatically break anything?

  Is there any documentation on either adding these, or whether it could be done at a domain level?

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  cPRex Jurassic Moderator

  • July 18, 2024 13:32

  I'm a big believer of "anything is possible" but performing this work on just one domain instead of all mail headers is even more complicated.  That would require a conditional filter that watches the sender domain and then applies custom headers based on that.

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  ITHKBO

  • July 18, 2024 14:38
  • Edited

  I am going to hazard a guess here that it might be possible by editing the /etc/exim_outgoing.conf and injecting the header there by putting it under a ACL as a extra line add_header = x-ms-reactions: disallow

  However Exim is very sensitive to wrong configurations so before even attempting such a thing my recommendation is to use a staging server with no clients on it except yourself.

  And again this is a guess. 
  
  Here is the exim4 main ACL information guide
  https://www.exim.org/exim-html-current/doc/html/spec_html/ch-access_control_lists.html

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