Beginning on March 20, 2019, if after multiple attempts, Outlook is unable to send or deliver a message sent by one of your users, a bounce message with details will arrive within 24 hours instead of 48.

Gradually rolling out in March 2019.

Currently, when a temporary routing error occurs, there are repeated attempts to send the message for 48 hours before the system returns a bounce message (or non-delivery report (NDR)) to the sender. This change reduces Office 365 message expiration timeout interval from 48 hours to 24 hours.

By reducing the message expiration timeout interval to 24 hours, senders will know sooner if we were unable to deliver their message, and can respond to the error in a more timely fashion. Note that this will only affect messages queued on Office 365 servers.

Message expiration times for messages queued on servers outside of Office 365 are not affected by this change.