If your mail server is operating as an open relay, or if one of your user’s accounts has become compromised and is being used to send spam, then it is possible that your server has been added to a global blacklist. This means that mail from your mail server will be rejected by hundreds if not thousands of other email providers.
You can check to see if your server has been blacklisted at the following URLs:
https://mxtoolbox.com/blacklists.aspx
https://www.dnsbl.info/
