Outlook 365 with outlook.com and external domain primary alias

Question

Hello everyone

I'm facing issues with the combination outlook 365 on windows (and os x!), outlook.com premium and my email address <firstname@lastname.com>* as primary alias.

• not my email, just an example

Before I explain my setup, here the situations where everything works:
Outlook.com in the browser (Chrome and Edge)
Outlook.com account in Outlook iOS-app
Outlook.com account in iOS mail/calendar/contacts/notes
Outlook.com account in Windows 10/11 mail, calendar, contacts etc.

My setup is the following:
outlook.com premium account firstname.lastname@harsh.com .com (non-primary alias)
primary alias firstname@lastname.com
mail forwarding *@lastname.com to firstname.lastname@harsh.com .com
lastname.com hosted @ ionos
cname autodiscover for lastname.com pointing to autodiscover.outlook.com (also tried autodiscover-s.outlook.com, same)
2FA activated

I don't use custom domain feature in outlook.com beacause it doesn't allow catch-all, but that's a feature im using for many years now and it's gonna be impossible to find every single alias I've used before (netflix@, amazon@ etc...)

And here's the major issue I'm facing at the moment and that makes it almost impossible for meh to use the outlook.com account:

I can't add it to my Outlook 365 on Win10/11 and use it.

I add a new account, type firstname@lastname.com, activate the manual checkbox, in the next window I pick outlook.com, i confirm the license stuff from autodiscer, a new windows pop's up where I have to enter my 2FA one-time-app-pw, I pick how long I want the messages to synchronize and the I get the success message that the account is added.

So far so good.

After a restart of outlook.com, I just get a message saying: "name cannot be matched to a name in the address list", I can click ok, and the account is not there.

I'm working on this for at least 2 months now. I can reproduce as many times as I want on different machines with different windows versions (10/11) and also different outlook.com accounts. I added registry entries, I deleted windows credentials for a million times, I deactivated 2FA, I chatted to the Outlook in app support but then the ticket got lost in my outlook and nobody had a solution.

ONCE I managed to add the outlook.com account to one outlook 365 installation. I was using it for couple weeks and everything worked fine, but now another issue popped up: mail receipients get my mails like this: "firstname@lastname.com on behalf of outlook_XYZ123XYZXYZXYZ123XYZ@harsh.com .com" which is unsexy as well. The default sender address is set to firstname@lastname.com, but the receipient can see the cryptic outlook address. This happens from iOS, web and outlook 365.

I simply don't understand why it doesn't work and how to contact someone from MS who is responsible for this. Is it an issue? Is it a bug? Is it a feature? The internet is full with complains about the same issues, but why isn't there a solutuion?

Please help :(

thanks in advcance and best regards
ACH