How may we change the default "Undeliverable..." subject and "wasn't found"-message auto-reply?

Hello chad oasiswm.org,

Welcome to the Microsoft Q&A and thank you for posting your questions here.

Problem

I understand that you would like to change the default "Undeliverable..." subject and "wasn't found"-message.

Solution

Unfortunately, in Office 365, you cannot directly modify the text of default Non-Delivery Report (NDR) messages, such as the one sent when delivery restrictions are configured on a mailbox.

You have alternative few options to do this as listed in the followings:

By creating Transport rule for custom reason.

• You will need to create a transport rule that adds a custom reason to the NDR. While this won't change the entire message, it allows you to provide a more user-friendly explanation.
• Configure the rule to "block the message" and add your custom reason.
• Keep in mind that the NDRs will still look somewhat unfriendly, but at least you can include a more informative message.

https://techcommunity.microsoft.com/t5/microsoft-365/change-undeliverable-email-messages-auto-reply/td-p/215345

Secondly, by creating inbox rule.

• Instead of setting a restriction on the mailbox itself, create inbox rules.
• Set up a rule that replies with a custom message whenever an email is received, except if it's from a member of the allowed group. By doing this you will be able to customize the auto-reply message to be more friendly.

Lastly, by adding policy tip.

• You will need to add a "Policy Tip" to the shared mailbox.
• Display a text like "This mailbox is only for its members" to users before they send an email.
• NOTE: This won't modify the NDR directly, but it will provide a proactive warning to users.

References

Send automatic replies (out of office) from Outlook.

How to Set Up an Out of Office Message in Outlook.

Accept Answer

I hope this is helpful! Do not hesitate to let me know if you have any other questions.

** Please don't forget to close up the thread here by upvoting and accept it as an answer if it is helpful ** so that others in the community facing similar issues can easily find the solution.

Best Regards,

Sina Salam