Using Outlook for iOS in business environments for email and scheduling
Hello, Nicola Miles
Thanks for your quick response and provide the detailed screenshot of the NDR!
According to the content of NDR, "Your message couldn't be delivered cause that the account domain does not exist".
It is very likely that a hacker has hacked your account, added his account as an alias and set it as the primary alias and deleted your account, although this is an outcome you do not want to face, in this case, if the account is deleted, it will not be recoverable!
Add or remove an email alias in Outlook.com - Microsoft Support
Thanks for your understand!
Best regards
Cece | Microsoft Community Support Specialist