I now get the Exim prompt when I telnet to port 25.

Code:
[root@rocky ~]# telnet mail.petrahigh.co.zw 25
Trying 79.170.44.54...
Connected to mail.petrahigh.co.zw.
Escape character is '^]'.
220 mail54.extendcp.co.uk ESMTP Exim Sat, 01 Dec 2012 01:20:10 +0000
ehlo rocky.example.com
250-mail54.extendcp.co.uk Hello rocky.example.com [10.10.10.10]
250-SIZE 31457280
250-PIPELINING
250-AUTH PLAIN LOGIN CRAM-MD5
250-STARTTLS
250 HELP
mail from: someone@example.com
250 OK
rcpt to: root@petrahigh.co.zw
550 unknown user
quit
221 mail54.extendcp.co.uk closing connection
Connection closed by foreign host.
Since I'd imagine root is a valid user, it may not accept mail for the petrahigh.co.uk domain yet.