365 email temporary delay from one source

This happened in the past and is no longer an issue. Status emails about planes are sent frequently and there was a period of time where those mails were delayed by hours. To try and include only pertinent information here is a couple of hops from one message header

Received: from flyht.aero (216.220.49.132) by
 YT3CAN01FT014.mail.protection.outlook.com (10.118.140.148) with Microsoft
 SMTP Server id 15.20.6363.21 via Frontend Transport; Tue, 2 May 2023 17:08:14
 +0000
Received: from ARIA [192.168.1.131] by flyht.aero with ESMTP
  (SMTPD-12.5.3.99) id f4ac00852b1614ec; Tue, 2 May 2023 07:38:46 -0600

The message was created at 5/2/2023 9:38:46 AM and sent to flyht.aero which passed it off to Microsoft SMTP Server at 5/2/2023 1:08:14 PM. There was several other MS servers it passed though before it reached the recipient but it was a matter of seconds.

My question is how do I interpret this delay. Was the delay in

• flyht.aero sending it? or
• Microsoft SMTP Server receiving it.

I am not certain because its not outlandish to think the MS throttled them on this day. I am ultimately trying to figure out if I have enough information to prove it was as unknown 3rd party issue of if our MS tenant was telling them to pump the brakes.

If the latter I need to know how to recognize that if it happens again or possibly "whitelist" this sender from throttling... which isnt likely possible. Any insight could be helpful.

If there aren't other hops in the header between flyht.aero and Exchange Online, the cause may possibly be with throttling.

If the issue happens again, on Exchange Online side you can run message trace for this delayed message and see if you can see this message in "Received" event:

A delay of xx seconds took place prior to Exchange Online.

And if you have access to the SMTP send logs of the flyht.aero server, you can check if you can see following response from Exchange online:

4.5.3 Too many recipients (AS780090) 4.7.500 Server busy. Please try again later from [XXX.XXX.XXX.XXX]. (S77719) 4.7.500 Server busy. Please try again later from [XXX.XXX.XXX.XXX]. (AS77712XXX) 4.7.500 Access denied, please try again later. For more information please go to http://go.microsoft.com/fwlink/?LinkId=526653 . (AS63)

If yes, the delay is caused by throttling in Exchange Online.

You can create a receive connector for this sender to get a much higher throughput before getting throttled. (By default if messages use the default connector, the throughput would be lower)