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Is it possible to create an ETR that blocks certain combinations of recipients? Imagine I have two customers in different domains with similar email addresses. I want to prevent sending to recipient lists that inadvertently mix users from the two domains.

Ideally, I'd block outbound messages with *@domainA.com AND *@domainB.com recipients. Also, blocking outbound messages with user@domainA.com AND *@domainB.com would be acceptable.

Consider also that the recipients list might be:

TO: user@domainA.com
CC: user@domainB.com

Exchange rule predicates seem to be "any" operators. Further, it seems I can't use the same condition twice; it's greyed out on the second condition.

Screnshot of Exchange Transport rule criteria

bendodge
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2 Answers2

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Is Exchange 2019 or Exchange Online?You could use “ address matches any of these words”, it can be added multiple domains:

enter image description here

Jayce
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  • Exchange Online. Wouldn't your proposed rule block emails with only a single recipient in either domain? – bendodge May 01 '23 at 19:48
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In Exchange Online, you also could add two domains into the “domain is”: enter image description here

To block outbound messages with user@domainA.com AND *@domainB.com, you could refer to the following settings:

enter image description here

Jayce
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