Ian,
The address you have is correct and I have just tested it and all appears to be fine as you will see from my log
Log:
MX record about STBbrackets.co.uk exists.
Connection succeeded to mail.STBbrackets.co.uk SMTP.
220 mail.stbbrackets.co.uk ESMTP MailEnable Service, Version: 1.986-- ready at 04/29/10 20:24:16
> HELO verify-email.org
250 Requested mail action okay, completed
> MAIL FROM: <check@verify-email.org>
=250 Requested mail action okay, completed
> RCPT TO: <contact@STBbrackets.co.uk>
=250 Requested mail action okay, completed
So, I must conclude that the problem is related to your ISP