Check the Message Events table below for any additional information about why message delivery might be delayed. For example, ...

Check the Message Events table below for any additional information about why message delivery might be delayed. For example, it might be due to a temporary issue trying to connect to the recipient's email server outside of Office 365. Many such delays clear up on their own, and the message gets delivered. If Office 365 isn't able to send or deliver the message within 48 hours, the sender will receive a non-delivery report (NDR) message with more information about how to fix the issue.

If you don't want to wait for the message to finish being processed, consider asking the sender to send the message again using a different email address.