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Microsoft 365 Exchange Email Catch-All

0 - Domains

Make sure all domains are set to Internal Relay, not Authoritative. ALL DOMAINS MUST BE SET TO THIS, otherwise none of this will work.
Do this from Mail Flow>Accepted Domains

1 - Shared Mailbox

Create a shared mailbox for the catchall messages to go to and set yourself as an admin. If you don't want to deal with a separate mailbox, you can set mail here to be forwarded to you.
Do this from Recipients>Shared.

2 - Dyn Dist List

Create a dynamic distribution list that will contain all valid users.
Do this from Recipients>Groups

3 - Mail Rule

Create a rule to send mail to the catchall mailbox if it is not sent to a valid address.
Do this from Mail Flow>Rules


IF the sender is located outside the org
DO redirect the message to the catchall mailbox
DO (opt) prepend a disclaimer/prepend to the subject line
EXCEPT IF the recipient is a member of the allusers group

Code for Disclaimer

/var/www/dokuwiki/data/pages/lab/m365-catch-all-email.txt · Last modified: 2021-05-06 02:24 by