Client host is blacklisted - Appears to be false positive

Hi. Recently, we are unable to send emails to any users using Sophos system. Keep on receiving bounced emails. We tried going to the sophos link to request delisting, it however show that the IP address is ok.

Appears to be a false positive. Can help to whitelist the IP address please?

We also tried calling sophos and was directed to submit an online form but its been many days we didn't get any reply. 

This is very urgent, hope Sophos can help, as we are unable to send any emails to Sophos users.

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This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

host mx-01-us-west-2.prod.hydra.sophos.com
SMTP error from remote mail server after RCPT TO:<email>:
554 5.7.1 Service unavailable; Client host [X.X.X.X] is blacklisted. Visit www.sophos.com/.../ip-lookup.aspx to request delisting