MS Mail servers claim Hotmail domain does not exist

In my Gmail account I have a rule that forwards everything to my Hotmail/Outlook account where I primarily read all of my mail. This has worked well for a long time until the 16th January 2026 when I started to realise I was missing a few mails. Looking in the Gmail inbox I could see a few NDRs from Googlemail saying that it had failed to forward a message. I'm assuming this is an issue at the Microsoft end as the NDRs say it is a remote (i.e. Microsoft) mail server claiming that the domain Hotmail.co.uk doesn't exist, so the mail gets bounced. 

BTW, this is not an issue with my individual account, my wife and I have similar setups (both our Gmails forward to our Hotmails) and we both started having the same issue from that date.

The key NDR details are always of this form:

Address not found

Your message wasn't delivered to ***@hotmail.co.uk because the domain hotmail.co.uk couldn't be found. Check for typos or unnecessary spaces and try again.

The response from the remote server was:

501 5.5.4 Invalid domain name [AM4PEPF00027A67.eurprd04.prod.outlook.com 2026-01-20T22:06:56.421Z 08DE543D7B61AE38]

So, it looks like some server in the chain is claiming that the whole domain "hotmail.co.uk" cannot be found!!! Surely I can't be the only one with this issue? I guess many people will be unaware they're missing mails as the original senders will be the ones receiving the NDRs, and where that is a company (rather than an individual) they probably just bin them and never investigate.

This seemed to get worse over a few days and then stabilise to ~25% of mails failing:

• 16th Jan - 3 of 31 emails had this issue
• 17th Jan - 0 of 19 failed
• 18th Jan - 3 of 16 emails had this issue
• 19th Jan - 1 of 34 emails had this issue
• 20th Jan - 5 of 27 emails had this issue
• 21st Jan - 10 of 34 emails had this issue
• 22nd Jan - 8 of 31 emails had this issue
• 23rd Jan - 8 of 28 emails had this issue

The issue seems to be random, there is no pattern with the times of day or senders - sometimes a particular sender is fine (e.g. Netflix, eBay), sometimes they suffer this issue.

In the NDRs I'm not sure what the naming convention of the Outlook servers is but re. the above example's "AM4PEPF00027A67.eurprd04.prod.outlook.com", I've looked at all the servers mentioned in each NDR (up to 22nd) and again there's no real pattern - the hostname's always begin with either AMS0, AMS1, AM3, AM4, DB1, DB3, DB5, DU2, DU6, with the 4 character ones being followed by "EPF000xxxxx" and the 3 character ones being followed by "PEPF000xxxxx". The following subdomain is always eurprd02-eurprd05 with just two examples using EURPRD83.

I did try and report to Outlook.com but their online help system currently gives an error (Error 500.30 - ASP.NET Core app failed to start). I hope this is a better place where someone might have a chance of convincing someone that there is a mail server config error somewhere?