If your mail server IP is blacklisted you would receive a bounce back error message when you send messages. Usually these will contain the reason and which service has blacklisted you. If you have doubts or need to check if your IP is being currently blacklisted by anyone you can use the MX Toolbox free tool here: http://mxtoolbox.com/blacklists.aspx Just type in your IP and it will check it for you. Below are the most popular de-list request pages:
SPAMCOP - https://www.spamcop.net/bl.shtml
SPAMHAUS - https://www.spamhaus.org/lookup/
BARRACUDA - https://www.barracudacentral.org/rbl/removal-request
LASHBACK - https://blacklist.lashback.com/
Once you determine which service has blacklisted you a request can be made to them to remove your IP.
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