421 Service Temporarily Unavailable

Hi,

A client using my server for emails is having issues sending emails to a client using your service. The errors are:

AD053E1804B0    15556 Thu Jun 22 17:04:22  i**@n****.es
(host cluster4a.eu.messagelabs.com[216.82.251.230] said: 421 Service Temporarily Unavailable (in reply to RCPT TO command))
                                         s***o@ca****nk.com

Our IP is 62.210.209.202 and it is not blacklisted.

If it is temporary like your error sugests how long will it take to heal itself?

Thanks in advance for any help/advice it would be much appreciated.

Kind Regards

Xabier