Replying to this one as it led to the solution.
Outlook 2003 was still failing authentication this afternoon, >24 hours since the first occasion.
I went to https://login.microsoftonline.com/
and tried to login. It responded that too many attempts had been made with an incorrect password and presented a login with Captcha. Logged in and was able to see the email waiting in the Inbox.
Then reset the Outlook password and, lo and behold, Outlook now connected to the email servers and downloaded emails.
What I now believe happened is that an authentication failure occurred (not that uncommon), and someone tried a different password, probably several times. In doing so Office 365 'locks' the account after a certain number of failed attempts and will only 'unlock' after a Captcha authenticated login, something that Outlook can't do.
I've now found
Scenario 4: The account is locked out
To determine whether an account is locked out, sign in to the Office 365 portal (https://portal.microsoftonline.com). You may receive a message that resembles the following:
You've tried to sign in too many times with an incorrect email address or password.
To resolve this issue, use one of the following methods:
* Wait 15 minutes, and then try again.
* Have the Office 365 admin reset the password. This unlocks the account.
* If you are signing in by using an incorrect password on a mobile device, update your password to the correct password on the mobile device.
Be aware of the following password lockout policies:
* After 10 unsuccessful sign-in attempts with an incorrect password, you will have to solve a CAPTCHA as part of the sign-in process.
* After another 10 unsuccessful sign-in attempts with an incorrect password and after you correctly solved the CAPTCHA, you'll be locked out for 15 minutes.
* Additional sign-in attempts with an incorrect password results in an exponential increase in the lockout time period.
I will educate the office folk at the resident's club accordingly.
We have more and more laws, and less and less enforcement