SMTP 554 5.7.1 with smarthost/relayhost

[Martin Campbell]

I am using Kolab with a smarthost/relayhost because my SMTP server has a 
dynamic IP address. Some SMTP servers block dynamic IP addresses.
Occasionally a recipient's SMTP server sends the following error message 
to the smarthost/relayhost SMTP server (which is then forwarded back to me).
"SMTP protocol diagnostic: 554 5.7.1 This message has been blocked 
because the return email domain is invalid"
Can anyone suggest a solution to this situation?
Regards,
Martin