I now get the Exim prompt when I telnet to port 25.
Since I'd imagine root is a valid user, it may not accept mail for the petrahigh.co.uk domain yet.
[root@rocky ~]# telnet mail.petrahigh.co.zw 25
Connected to mail.petrahigh.co.zw.
Escape character is '^]'.
220 mail54.extendcp.co.uk ESMTP Exim Sat, 01 Dec 2012 01:20:10 +0000
250-mail54.extendcp.co.uk Hello rocky.example.com [10.10.10.10]
250-AUTH PLAIN LOGIN CRAM-MD5
mail from: email@example.com
rcpt to: firstname.lastname@example.org
550 unknown user
221 mail54.extendcp.co.uk closing connection
Connection closed by foreign host.