This is the error message.
An unknown error has occurred. Account: 'mailhost.zen.co.uk', Server: 'mailhost.zen.co.uk', Protocol: SMTP, Server Response: '550 You're not allowed to talk to me', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC69
I have had at least seven sent e-mails that seem to have vanished in the past two weeks.
Guess I will have to duplicate the important ones and send them via dial up until things are sorted.