216.82.251.230 RCPT TO: 421 Service Temporarily Unavailable

I need a solution

 
Hello.
I am the administrator of the mail server windows-plesk.comel-it.com
Recently, one of our clients reported that they could not send messages to messagelabs.com customers.
Our server obtains the message as in the post title. The IP of our server is 78.28.40.175 and we are not listed on any blacklist.
Please tell us what steps we have to make to make the small pass properly.
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