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QasimSheikhM-1956 asked YukiSun-MSFT commented

Exchange Rule - Mailbox doesn't exist

How can I add a rule in MS Exchange Online, that, if the mailbox or email address does not exist, instead of bouncing back, it should just go to a particular user/mailbox?

GSuite has this feature, is there a way to implement this on M365? Also, some users have been deleted as they do not exist, but emails are coming to them.

office-exchange-server-administrationoffice-exchange-server-mailflow
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Hi @QasimSheikhM-1956

I am writing to see how everything is going on with this thread. Have you had a chance to check the replies provided earlier? If you still have further concern on this, please feel free to let us know.


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Apologies, I didn't get the chance to do this earlier! Yes, it worked and I have accepted the answer! It has been a while since I have worked on MS Exchange and I totally for about the relay bit.

Disregard: "Then regarding the other issue you mentioned (some users have been deleted as they do not exist, but emails are coming to them), since it's different from the initial question as indicated in the thread title, it would be best if you try to open up a new thread for it. In this way, it will make information searching in the forum easier and be beneficial to other community members as well. Thanks for your understanding."

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YukiSun-MSFT avatar image
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YukiSun-MSFT answered YukiSun-MSFT commented

Hi @QasimSheikhM-1956

How can I add a rule in MS Exchange Online, that, if the mailbox or email address does not exist, instead of bouncing back, it should just go to a particular user/mailbox?

For Exchange Online environment, Directory-Based Edge Blocking (DBEB) is by default in effect, so messages for invalid recipients will be automatically rejected at the service network perimeter and won't be able to go through the rest of the service filtering layers: anti-malware, anti-spam, and mail flow rules. Therefore, in order to catch mails sent to nonexsiting mailboxes, first we would need to disable DBEB by changing the domain from Authoritative to Internal relay, then try to create a transport rule to redirect the messages.

Below are the steps for your reference:
1.Login to Exchange Admin Center, go to Mail Flow > Accepted domains, choose the domain, change it from Authoritative to Internal Relay:
130919-1.png
For more details about the difference between the authoritative domain and internal relay, you may refer to Manage accepted domains in Exchange Online.

2.Go to Recipients > Groups, create a group which includes all the valid internal recipients. For instance, you can create a dynamic distribution group and set the rule as all recipient types so that all valid recipients will be included.

3.Go to Mail Flow > Rule, create a rule similar to below:
133708-rule.png

Then regarding the other issue you mentioned (some users have been deleted as they do not exist, but emails are coming to them), since it's different from the initial question as indicated in the thread title, it would be best if you try to open up a new thread for it. In this way, it will make information searching in the forum easier and be beneficial to other community members as well. Thanks for your understanding.


If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.



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The only other addition to the exceptions was to include if "The send is a member of" the same group. Otherwise, all emails sent out landed in the catcher mailbox. So the rule was to redirect all messages and except that is sent or received by an active account/user.

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YukiSun-MSFT avatar image YukiSun-MSFT QasimSheikhM-1956 ·

Hi @QasimSheikhM-1956

Thanks for pointing this out and sorry for the negligence in my previous post. Aside from adding the exception as you mentioned, we can also change the rule condition to make the rule apply only if senders are located outside of the organization:
133784-rule.png

I'll edit the rule screenshot in my reply earlier in case it cause confusion for others who read this thread in the future.

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rule.png (30.1 KiB)

Yes, thank you, that will be a better ruleset. Thank you for that.

Also, yes, please, for other users who may have the same issue, it will be very helpful. Thank you!

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michev avatar image
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michev answered

If that's for specific (previously existing) email addresses, simply add those as aliases to some shared mailbox or a DG. If you are talking about a general "catch all" type of solution, read here: https://www.undocumented-features.com/2021/02/10/create-a-catch-all-mailbox-in-office-365/

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