Hello community. Please tell me, can I implement a mail rule that will intercept a message if there are more than 20 recipients in the recipients (To) field? I would like to do the same thing in Outlook. GPT claims that it is possible and even says how, but this is just his imagination :) Thank you.

Hello, @ Mountain Pond , Welcome to the Microsoft Q&A platform! Yes, you can implement a mail rule in Outlook to intercept messages with more than 20 recipients in the "To" field. This can be done in Exchange Admin Center (EAC). Here’s a general outline of how you can set this up: 1.Log in EAC, visit Recipients -> Mailboxes and click on your mailbox. Select Others -> Set recipient limit in the pop-up screen as the screenshot shown below. 2.Change Maximum recipients from the default of 500 to your requested 20 as the screenshot shown below. 3.During my configuration, to make it easier to test, I set the recipient limit to 1 and tried to send a mail to 2 recipients at one time. The email was blocked as expected because the number of recipients exceeded the maximum number. Hope the solution above can help you reach your goal. Should you need more help on this, you can feel free to post back. If the answer is helpful, please click on ACCEPT ANSWER as it could help other members of the Microsoft Q&A community who have similar questions and are looking for solutions. Thank you for your support and understanding. Best Wishes, Alex Zhang

Prevent message delivery if there are multiple recipients in the To field

Accepted answer

Alex Zhang-MSFT 3,005 Reputation points Microsoft Vendor

2024-12-19T02:30:02.6+00:00

Hello, @Mountain Pond,

Welcome to the Microsoft Q&A platform!

Yes, you can implement a mail rule in Outlook to intercept messages with more than 20 recipients in the "To" field. This can be done in Exchange Admin Center (EAC). Here’s a general outline of how you can set this up:

1.Log in EAC, visit Recipients->Mailboxes and click on your mailbox. Select Others->Set recipient limit in the pop-up screen as the screenshot shown below.

2.Change Maximum recipients from the default of 500 to your requested 20 as the screenshot shown below.

3.During my configuration, to make it easier to test, I set the recipient limit to 1 and tried to send a mail to 2 recipients at one time. The email was blocked as expected because the number of recipients exceeded the maximum number.

Hope the solution above can help you reach your goal. Should you need more help on this, you can feel free to post back.

If the answer is helpful, please click on ACCEPT ANSWER as it could help other members of the Microsoft Q&A community who have similar questions and are looking for solutions.

Thank you for your support and understanding.

Best Wishes,

Alex Zhang
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1 person found this answer helpful.
Mountain Pond 1,481 Reputation points

2024-12-19T17:54:53.7133333+00:00

Sorry, but I don't need to limit the number of recipients for my users. I need to intercept an incoming message from an external sender that has multiple recipients in its To field.

Alex Zhang-MSFT 3,005 Reputation points Microsoft Vendor

2024-12-20T02:42:07.3433333+00:00

Hello, @Mountain Pond,

According to my check and test, unfortunately, as of the last update, Exchange doesn't natively support checking the exact number of recipients directly in a transport rule. In the “To” field, we can specify specific recipients or recipients containing specific information. If you delete emails according to the number of recipients, it is easy to delete emails by mistake.

If you are trying to get rid of the hassle of receiving a lot of mass emails, you may consider using Mail Flow Rules or Anti-Spam Policies to achieve this goal. For more instructions, please refer to:

https://learn.microsoft.com/en-us/exchange/security-and-compliance/mail-flow-rules/use-rules-to-filter-bulk-mail.

https://learn.microsoft.com/en-us/defender-office-365/anti-spam-policies-configure.

If the answer is helpful, please click on ACCEPT ANSWER as it could help other members of the Microsoft Q&A community who have similar questions and are looking for solutions.

Best Wishes,

Alex Zhang

Mountain Pond 1,481 Reputation points

2024-12-20T11:53:44.14+00:00

In fact, one person is targeting many users in an organization, sending unpleasant and offensive messages, different each time. It uses the To field, which contains multiple email addresses. Which would allow these messages to be identified.

Alex Zhang-MSFT 3,005 Reputation points Microsoft Vendor

2024-12-23T02:40:19.92+00:00

Hello, @Mountain Pond,

I understand the frustration of receiving mass offensive emails. Please excuse the fact that for security reasons, Microsoft's rules do not allow us to limit the number of recipients for accounts other than those under your control at this time.

I suggest that you either block the account that is sending the offensive emails directly or consider the two alternatives I've given above.

Best Wishes,

Alex Zhang

Mountain Pond 1,481 Reputation points

2024-12-24T19:26:09.7833333+00:00

Thank you @Alex Zhang-MSFT

I analyzed the letters that are sent. In the "To" recipient field, there are many recipients from different organizations, ~400 recipients, and only a small part of the recipients are from our organization.

I created a rule that marks the message as SPAM if the sender is from outside and if in the header in the "To" field there are some addresses that do not belong to our domain.

If there are any other options, please let me know.

The sender always uses different addresses from gmail for example.
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