# Microsoft 365 Exchange Catch-All Email

Application: O365
Category: Guide

[Exchange Admin Centre](https://outlook.office365.com/ecp/)

## 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.

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## 2 - Dyn Dist List

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

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## 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

[https://gist.github.com/itsmeimtom/d5fb2eb3c4636d263e463f91efeebf5a#file-catchall-html](https://gist.github.com/itsmeimtom/d5fb2eb3c4636d263e463f91efeebf5a#file-catchall-html)