Mail Add-on

Mail Add-on

I cannot for the life of me get the Mail Add-On to accept my POP3 details.  I am certain I have them right but when I try I get the authentication error.  That's diagnostic of an incorrect password, but I know that's correct (as I am sure are all the other settings).  I even reset the password to make sure!

Can anyone give me any clues as to what else might trigger this error message?