You can follow a procedure at spamhous.org to remove you from the blacklist, if the reason is that you have an open relay - a common issue.

An open relay is where someone can send mail to your mail server addressed to others and your server will relay the messages to the addressee. The addressee will see that it cam from you, not the original sender. Thus, you become a spammer.

Most email servers automatically contact the several spam blacklist websites to automatically block spam.

In my own case I went from more than 1,100 spam messages addressed to me per day to zero in over 6 months since implementing the spam blocking feature that is part of Microsoft Exchange Server.

My mail server services only myself and my family and is based in my home.