Can't send email to 1 particular client - "Client host rejected"

[ChrisMarin]

hi guys,

we get the following return message when trying to email holidays@***********.co.uk:

>>> RCPT To:<HOLIDAYS@***********.CO.UK>
<<< 550 5.7.1 Client host rejected: cannot find your hostname, [***.***.**.***]


I have checked Merak Mail Server's log and it states the follwing:

63.251.171.168 [0DE0] 12:14:25 Client session Connected
63.251.171.168 [0DE0] 12:14:26 Client session <<< 220 mail7.zoneedit.com ESMTP Postfix
63.251.171.168 [0DE0] 12:14:26 Client session >>> EHLO smtp.eclipse.co.uk
63.251.171.168 [0DE0] 12:14:26 Client session <<< 250 DSN
63.251.171.168 [0DE0] 12:14:26 Client session SSL: Not verified (6) - proceed anyway
63.251.171.168 [0DE0] 12:14:26 Client session >>> MAIL From:<data@***********.co.uk> SIZE=3005
63.251.171.168 [0DE0] 12:14:27 Client session <<< 250 2.1.0 Ok
63.251.171.168 [0DE0] 12:14:27 Client session >>> RCPT To:<HOLIDAYS@***********.CO.UK>
63.251.171.168 [0DE0] 12:14:27 Client session <<< 550 5.7.1 Client host rejected: cannot find your hostname, [***.***.**.***]
63.251.171.168 [0DE0] 12:14:27 Client session >>> QUIT
63.251.171.168 [0DE0] 12:14:27 Client session <<< 221 2.0.0 Bye
SYSTEM [0DE0] 12:14:27 Client session Delivery error response error
63.251.171.168 [0DE0] 12:14:27 Client session >>>
SYSTEM [0DE0] 12:14:27 Client session *** <> <> 0 0 00:00:00 ERROR
SYSTEM [0DE0] 12:14:27 Client session Disconnected


Any ideas why this isn't working? 98% of all other emails are fine.

Thanks

Chris.