New Mail account constant nag for password


I’m using Yosemite and Mail on a number of Macs and setting up mail for a host is always a pest. Regardless of what you do it always nags for the password then miraculously stops and works. With one iMac it refuses to accept.

It originally said ‘Internet Accounts preferences can’t connect to the acount XXX’. On reboot it now says ‘Mail can’t connect to the account XXX’.

It can’t connect to the POP server.

Log says: [kCFStreamSocketSecurityLevelTLSv1_0]
-ERR [AUTH] Authentication failed.

Everything is exactly the same as my Mac Air but its playing this game. Password is OK, checked host settings etc etc.

I’ve deleted the certificate and checked that everything the new one is set to always accept connections. Is there a prefs file I can delete to fix this? I could also copy relevent files over from the Mac Air if I knew which ones to copy across.