Every undelivered message in Exchange Online will be re-sent until it reaches its intended recipient. Here, Message Expiration is introduced among organizations to prevent undelivered messages from being resent again after a certain interval and notifies the sender with a non-delivery response message.
This message retrying option has been around for a while and is well-liked by businesses. By default, Microsoft has set the value to 1 day (24 hours). It'll continuously retry to send the message for one day. However, sometimes one-day intervals are often too long to notify users that their messages aren't delivered!
So, in response to this, Microsoft has now made the message expiration interval period to be customized by admins.
Now, admins can customize the message expiration timeout and set a value between 12 to 24 hours.
You can now set the message expiration timeout interval for your organization as suggested below:
https://m365scripts.com/exchange-online/customize-undelivered-message-retry-period-in-exchange-onlin...
