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.
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