We're seeing problems delivering to MS customers now. I'm not sure if it's they are full, busy, or perhaps rate-limiting to try and slow the deluge.

Last Error: 451 4.7.0 Temporary server error. Please try again later. PRX3 NextHop: substrate-int.office.com. Last IP: [MI3PEPF00008491.eurprd03.prod.outlook.com 2026-11-13T10:15:37.721Z

Last Error: 421 4.3.2 The maximum number of concurrent server connections has exceeded a limit, closing transmission channel [MI3PEPF00008487.eurprd02.prod.outlook.com 2026-11-13T10:42:55.252Z ]

We're also seeing messages about alternate failover failing.