Delays when sending or receiving emails

Hi all,

Multiple customers are experiencing huge delays when sending and/or receiving emails.
Emails eventually are successfully delivered to inboxes or quarantined.

Example message trace sender:

Received -> Processed -> Sent
12/12/2023, 5:22 PM | Receive (with external delay) | Message received by: AS8PR03MB7253.eurprd03.prod.outlook.com using TLS1.2. There was a delay of 25 minutes prior to the message being received by Office 365.
12/12/2023, 5:22 PM | Send external | Message sent to xxxxxxxx-xxxx.mail.protection.outlook.com at 52.101.73.1 using TLS1.2 with AES256

Example message trace external recipient:

Received -> Not Processed -> Not delivered
Status: Unfortunately, we aren't able to provide an analysis for this message at this time.
More information: You can review the Message Events table below for details about the status of this message.

12/12/2023, 5:22 PM | Receive | Message received by: AM9PR04MB8777.eurprd04.prod.outlook.com using TLS1.2 with AES256
12/12/2023, 5:22 PM | Defer | Reason: 400 4.7.721 Advanced Threat Protection scanning in progress.
12/12/2023, 5:23 PM | Receive | Message received by: AM9PR04MB8777.eurprd04.prod.outlook.com
12/12/2023, 5:23 PM | Drop | Reason: [{LED=550 4.3.2 QUEUE.TransportAgent; message deleted by transport agent};{MSG=};{FQDN=};{IP=};{LRT=}]
12/12/2023, 5:23 PM | Spam | No detail information available.
12/12/2023, 5:23 PM | Defer | Reason: 400 4.7.721 Advanced Threat Protection scanning in progress.

I just raised a Microsoft support case and will keep you all posted.