Sounds like the email admin at the other end has blocked you from sending emails to them.
What domains are the bounce backs coming from?
Use other DNS testing sites in addition to CheckDNS. For example, try dnsstuff.com. If results still come back negative, then it’s possible that your messages are being rejected not because of a real-time black list, but because your IP address or subnet is explicitly defined on the list of blocked addresses on the recipient’s server.
Did you relay via other server (SMTP and mai box are not the same server)? If you did not send mail direct from your mail server, you may get that error. That is what we called “Challenge Response”.
The Challenge Response system requires senders from external domains to authorize their email address for future communication with your mail server. External senders will only be challenged once.
When the receiver’s server get your mail (firstname.lastname@example.org), it will send another mail to your SMTP server to check if that email (email@example.com) is valid. If firstname.lastname@example.org is not on that SMTP server, the receiver will reject your mail and your will get 550 5.7.1 Requested action not taken: message refused.