[SOLVED] Microsoft 365, how to setup a single user with multiple emails on different domains?

Microsoft 365, how to setup a single user with multiple emails on different domains?

On MS 365, for a single user I'd like to setup two real emails: username@mydomain.com and username@myotherdomain.com.

user@mydomain.com is well setup. Now I'm trying to add username@myotherdomain.com.

So far I tried to add username@myotherdomain.com

as an aliases but it's not a real email
as a distribution list that can receive email from outside the organisation and where user@mydomain.com can "Send As". It doesn't work. I tried to send an email to it via gmail and it failed and I tried to send an from it but it said that I don't have the "on behalf rights" (I've setup the "Send as" rights not the "on behalf of" ones...)
as a shared mailbox. It almost worked. I does with anyotherusername@myotherdomain.com but not with username@myotherdomain.com

Does anyone know how to achieve what should be a simple task?

Solution

I realized that when I create username@myotherdomain.com, it automatically tries to create an alias username@mydomain.com. And this is why it fails. So I found the following workardound that works.

I create username2@mytoherdoamin.com and save
I edit username2@myotherdomain.com to delete the aliases (including username2@mydomain.com then) and update the email to be username@myotherdomain.com and I save.