certificate of mail stalling

I have Mail on my Macbook Air and get mail from an account and on my other machines there is no problem getting that mail but on my Air there's a message that pops up:

The problem is the Connect button doesn't work - the whole program stalls and none of the buttons lets me get back into Mail. This account is fine on my other machines but this Air copy doesn't work and stalls Mail. I have repaired permissions and also restarted the computer but it still happens.