Outlook email account can receive mail but loses authentication and stop sending

I have been setting up a PC with Thunderbird for my mother. I want to move her off a "Messaging Company" email to an Outlook email address. Currently I have 3 email accounts on her PC: - Messaging Company - Outlook - Gmail (loaded for testing and because she has an Android phone) Messaging Company and Gmail seem to behave just fine, both for sending and receiving. Outlook seems to work fine for receiving email. For sending, it works when I clear the credentials and set them up again. This only seems to last for a while, and it's as if Thunderbird forgets the SEND credentials. I'm not sure if this is after a reboot or some other cause. I have walked through all of the relevant advice on OAuth2, STARTTLS, etc. I don't think it is a configuration issue at this level because I can send for a while. It seems to be something to do with retaining the credentials. Any help would be appreciated - if I can't get this going, then I may have to move here to the Gmail account but I have my own reasons for wanting to use Outlook.