If you are seeing email bounce with an error like this;
2016-02-18 03:43:14 188.8.131.52 OutboundConnectionResponse SMTPSVC1 WIN-DP0EEHPE3HP - 0 - - 521+5.7.1+Service+unavailable;+client+[46.nn.nn.7]+blocked+using+prs.proofpoint.com 0 0 83 0 5969 SMTP
Then you will need to request a delist of that IP address at the following URL;
Please be sure to run a malware scan and check that the server at this IP address is not sending spam or other bulk email (like marketing email) before requesting a delist.
Fill in the IP Lookup form at the top of the page to see if the address is listed, and if it is complete the next form presented to you telling them via the Additional Comments field that you have checked and secured the server, assuming that you have!