Hi @Oliver Lennox ,
Usually for Office365, it will be with the email address to be entered at the credential prompt. If the prompt occurs with olddomain\email@mathieu.company .com then can you check the below,
- Is that device still joined to old domain
- Any cached credentials in credential manager with olddomain reference
- How the users are logging in to the device (domain\username or emailaddress)?
Does it happen only while creating a new outlook profile?
Also, any recent changes as you have mentioned that it was working fine for several years?
Ideally, if the AD is removed, device should have work or school Microsoft account in the local accounts and login will be with email@mathieu.company .com.
If the above suggestion helps, please click on "Accept Answer" and upvote it