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Configuring Catch all Mailbox in Office 365 | Part 2#2

In the current article, we will complete the last step in which we create the required Exchange Online transport rule that will implement the concept of the “catch all mailbox.”

Just a quick reminder – in the former article, we have created a dynamic distribution group named – “All of Office 365 recipients” that include all of our organization recipients.

Also, we have changed the default setting of our domain name that registered at Exchange Online (o365pilot.com) to the internal relay instead of iterative.

  • In step 3 we will create the “catch all” transport rule.
  • In step 4 – we will test the “catch all” transport rule to make sure it is working correctly.
Note – this is the continuation of the former article – Configuring Catch all Mailbox in Office 365 | Part 1#2

Step 3 – Creating a Catch all Exchange Online transport rule

In this phase, will create a new Exchange Online transport rule

  • Login to Exchange Online admin center
  • On the left bar menu, choose the mail flow menu
  • On the top bar menu, choose the rules menu
  • Click on the plus sign and choose the menu – Create new rule…

Creating a Catch all Exchange Online transport rule -01

The “little secret” regarding the Exchange transport rule is that the default wizard interface includes only a limited option.

In case that we need to use the “advanced interface” of the Rule wizard that includes an additional option that we will need for creating our “catch all” rule, we need to choose the More options… menu

Creating a Catch all Exchange Online transport rule -02

In the Name: box, add your name.

In the following screenshot, we can see the basic structure of the Exchange Online rule.

  • The first box relates to the condition to be held.
  • The second box refers to the action that will be executed by the Exchange Online rule.

Creating a Catch all Exchange Online transport rule -03

In the Apply this rule if… box, choose the menu – The Sender… and in the sub menu choose is external/internal

Creating a Catch all Exchange Online transport rule -04

In the select sender location select – Outside the organization

Creating a Catch all Exchange Online transport rule -05

In this step, we will set the “action” that will be taking place.

In the *Do the following…. Box select the menu Redirect the message to… and in the sub menu choose these recipients

Creating a Catch all Exchange Online transport rule -06

In our particular scenario, we will use “Bob mailbox” as the “Catch all mailbox“.

From the recipient list, we will choose the required recipient who will be utilized for “Catch all mailbox” and click on the add-> button.

Creating a Catch all Exchange Online transport rule -07

Phase 3 – Define the exception part

This is the “third phase” in which we define the exception. This part is a little tricky from the “logic point of view perspective”.

To make it simple, in this phase, we just “say” to Exchange Online that if someone send E-mail using an E-mail address of a recipient which is not included in our organization recipient list, the E-mail will be “redirect” to the catchall mailbox.

The organization recipient list is defined by using the Dynamic distribution group that was created in the previous article.

In the Except if… box, click on the adds exception button.

Creating a Catch all Exchange Online transport rule -08
Choose the menu The recipient… and in the sub menu choose – is a member of this group

Creating a Catch all Exchange Online transport rule -09

In our particular scenario, the Dynamic distribution group that includes all of our organization recipient named – All of Office 365 recipients

Creating a Catch all Exchange Online transport rule -10

In the following screenshot, we can see the “final result” of the Exchange Online rule.

The rule consists of three parts

Creating a Catch all Exchange Online transport rule -11

Step 4 – Testing that catch all rule

In this part, we want to test the “catch all mailbox” rule that we have created.

The “test scenario” will be implemented by sending E-mail message to a “non-existing recipient” from the domain o365pilot.com

In our specific scenario, we will use Suzan’s mailbox for sending E-mail message to the following E-mail address – az#455@o365pilot.com

Testing the Exchange Online Catch all rule -01

In the following screenshot, we can see Bob’s mailbox.

We can see that although the E-mail address az#455@o365pilot.com “doesn’t belong to Bob, the E-mail message was sent to Bob mailbox be the “help” of the Exchange Online transport rule.

Testing the Exchange Online Catch all rule -02

If we want to check the “flow” of the E-mail message, we can use the Exchange Online message trace tool for finding information about the particular E-mail message.

In our specific scenario, we will look for E-mail that was sent by Suzan.

Testing the Exchange Online Catch all rule -03
In the following screenshot, we can see that the E-mail message that was sent by Suzan to the recipient az#455@o365pilot.com was “redirected” to Bob’s mailbox.

Testing the Exchange Online Catch all rule -04

The o365info Team

The o365info Team

This article was written by our team of experienced IT architects, consultants, and engineers.

This Post Has 6 Comments

  1. Thank you for explaining that, It was very easy for me to apply it after reading your post.

    Thanks!

  2. Great article. But I have a big problem. I have already removed this rule but still receiving hundreds  emails.. How to stop it?

  3. Great article that finally resolved the issue of unknown recipients. Great work! Thank you!

  4. Good article and it really works for mail adresses, which are not found on the Exchange Online Server, but it does not catch ALL mails.

    The point is, that rejected mails were not forwarded into the catch all Mailbox. For example if there is an o365-group which does not permit external senders. Those mails are not being collected in the catch all Mailbox.

    Do you have an idea, what I can do to make that happen as well?

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