Issue
I have sent an external Out of Office reply but is returning the following error.
We have found this to be a Microsoft issue, which was reported on the Microsoft status page on February 26th.
This only affects external out-of-office replies.
451 Temporary server error. Please try again later. 451 4.4.4 Mail received as unauthenticated, incoming to a recipient domain configured in a hosted tenant which has no mail-enabled subscriptions....................
Solution
For more information, please refer to the Microsoft service status page: "ID:EX719348: Some users' outbound Exchange Online email messages may be marked as spam and not delivered.