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Catch-All Account on O365

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Jose Gabriel Ortega Castro
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MCP / MCSE Messaging and Infrastructure
How to create a "CatchAll" mailbox account using a Shared O365 mailbox and a "Dynamic Distribution Lists" to exclude the internal results.

The goal of the following steps is to create a “Catch-All” account that no matter how many domains you have on your O365 Tenant, it receives all emails with the primary goal of not to lose any emails.

This process is not supported by Microsoft support and only works for Full Cloud Tenants. That said, let’s proceed to the steps:


1. Create a Shared Mailbox


To avoid the creation of a new “paid” mailbox with a license. We are using a “Shared Mailbox” called “Catchall@faboit.com” for receiving all emails.

Go to Office 365 Portal > Groups > Shared Mailboxes > Add a Mailbox



We use “Catchall2” because I already had one account with “Cathall” as UPN. Focus on the message that can take a few minutes and then click “add members to this mailbox.”

On the next steps, you need to add a person, a group, or anyone that you want to receive all misleading emails; with this, we finish phase 1 of 4.

2. Create a Dynamic List


On this new part, we are creating a new Dynamic List from the Exchange admin portal.

Go to Office 365 Portal > ShowAll


Show All / Finally, Go to “Exchange”


Open Exchange Administration Panel / On the exchange administration panel

Go to Recipients > groups > little triangle > Dynamic Distribution list


Fill out the “Name” and “alias” fields with: “allusers,” select a user to the owner and manage the group and then click “Save.”


A new Dynamic distribution is created. Now, open your newly created “dynamic distribution list” and make sure that on Membership is selected “All Recipients Types.”


Make sure that “All Recipients types” is selected and select “save.”

With the previous steps, the task is completed

3. Configure your domain (or domains) as Internal Relay


The third step is also on the Exchange Administration panel so go to:

Mail flow > Accepted Domains


Locale “Accepted Domain” on the EAP

Make sure that all your domains are selected as “Internal Relay,” if you fail to add this step, the transport rule will fail.

Internal Relay for mental peace.

This will finish the task 3 of 4.

4. Create a Transport Rule


The last part would be to create a transport rule on your Exchange Admin Panel,

Mail Flow > Rules > + > Create new rule


Rule Name: CatchAll Rule

if the sender is located “Outside the organization.”

Do the Following: Redirect the message to “CatchAll” (group on step 1).

Except if “The recipient is a member of”: “Allusers” (Dynamic distribution group on step 2).

Hit save and enjoy.


The CatchAll rule looks and feels.


Thank you very much for reading, any comments you can Private Message Me via my Experts Exchange Profile page.

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