There is a feature called 'Catch all' that is not supported by Microsoft Exchange Online. This feature ensures that no email sent to your organization has a Non Delivery Report(NDR) status.
So here is the thing with ' catch all' (no pun intended): usually, senders can omit some characters or make a mistake when entering email addresses, and this would ordinarily lead to a Non-Delivery Report (NDR) as the email address entered was either incorrect or does not exist.
What I mean is that a sender may type Cain@contoso.com instead of Caine@ncontoso.com. This will return an NDR. To capture and make sure such mails deliver and do not return an NDR, most mail providers provide the catchall mailbox. Now this is an unsupported feature in Exchange Online, but there is a workaround for it. The following explains how:
Set up the catchall mailbox (it is preferable to use a shared mailbox to conserve licenses, but user mailbox works as well)
Create a security group and add other mailboxes so they are separated from the catchall. This is so that we can exclude the other mailboxes from redirecting to the catchall mailbox.
Go to the Exchange admin center. Click on Mail flow> Accepted domain and select your default email domain. Set it as internal relay.
- Check the box, and new page opens up for you to select the 'internal relay' option then save.
- Click on Mail flow >Rules >Add a rule
- Create a new rule
- Give the rule a name, and give it the following conditions:
i. Apply this rule if : The Recipient + domain is (you specify your domain)
ii. Do the following : Redirect the message to + these recipients (you specify the catchall mailbox)
iii. Except if: the recipient + is a member of this group (specify the name of the group in step 2 above)
iv. Click on Save.
You can test your emails now and see that emails sent to wrong or nonexistent mailboxes on your domain are all caught by this mailbox.
I hope this helps someone.
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