Create Transport Rule.

Question

I have a Distribution List (DL) (dl1@contoso.com) in Exchange Online with 30,000 internal users. Only user1@contoso.com is allowed to send emails to this DL (Delivery Management: Specified Senders: user1@contoso.com).

User1 is planning to send an email to this DL with the subject "Important Email," and the DL will be in Bcc. My requirement is that none of the DL members should be able to reply to user1@contoso.com with subjects like "RE: Important Email" or "Important Email."

Will the following transport rule work?

If any user replies back by changing the email subject, then this transport rule will not work. Is there any other way to restrict users from replying to user1 to this one email even if the email subject is changed? Can we create a rule based on the message ID once the email is triggered?

Answer 1

Hi，@Glenn Maxwell

Thanks for posting your question in the Microsoft Q&A forum.

Based on your description, you want that none of the DL members should be able to reply to user1@contoso.com with subjects like "RE: Important Email" or "Important Email."

I will answer your questions in turn:

1. You created a rule that doesn't work as expected, this rule prevents anyone in your organization from receiving emails with the subject 'RE: Important Email' or 'Important Email.'

I created a new rule based on your request and tested it and it works as expected.

Because each email has a different ID and ID cannot be obtained automatically, it is not possible to create a rule to block sending based on the ID.There is no other way to restrict users from replying to user1 even if they change the subject of the email.

If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.