Presumably some sort of server problem, it's a pity that the response always seems to suggest that the password is wrong.
Receiving' reported error (0x800CCC92) : 'Your e-mail server rejected your login. Verify your user name and password in your account properties. Under Tools, click E-mail accounts. The server responded: -ERR Authentication failed.'
Edit: Sorry, I did not originally clarify this is not a problem with the Plusnet email server - it's an account with another provider. I was just having a bit of a moan because I know the message frightens a lot of people into entering their password into the pop-up windows and then trying 'alternatives' when the first attempt fails to work. There has got to be a better system.