"Trying to log in to this POP account failed"

I've set up a POP account in the Mail app and my details are getting rejected so I can't access my email.

I'm also being asked by a pop-up box in Mail to reneter my password and check the 'save in Keychain' box, which I've done many times to no good effect.

I've triple-checked(!) the port settings, password, username and online settings (the account is active and verified by my emai provider). I also relaunched Mail and did repair permissions FWIW...

Please advise.

(No threadjacks please).