421 Service Temporarily Unavailable

Hello,

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

2017-04-05 19:42:04 1cvpc4-000YoU-Td H=cluster1a.us.messagelabs.com [85.158.139.103]: SMTP error from remote mail server after RCPT TO:<ganesh.shankar@sita.in>: 421 Service Temporarily Unavailable
2017-04-05 19:42:04 1cvpc4-000YoU-Td == ganesh.shankar@sita.in R=dkim_lookuphost T=dkim_remote_smtp defer (-44) H=cluster1a.us.messagelabs.com [85.158.139.103]: SMTP error from remote mail server after RCPT TO:<ganesh.shankar@sita.in>: 421 Service Temporarily Unavailable
2017-04-05 19:42:04 1cvpc4-000YoU-Td == anurag.tiwari@sita.in R=dkim_lookuphost T=dkim_remote_smtp defer (-18) H=cluster1a.us.messagelabs.com [85.158.139.103]: Remote host closed connection in response to pipelined RCPT TO:<anurag.tiwari@sita.in>

SMTP error from remote mail server after RCPT TO:<ganesh.shankar@sita.in>: 421 Service Temporarily Unavailable

Can you see if our IP or their ISP IP has been temporarily blacklisted or why their emails are not getting through please??
server address is 80.172.253.133 and 80.172.253.139. 

Sender IP:

62.28.231.222

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

Mark