spam marked - Why am I getting 500 error while trying to send emails to some domains?

Recently I've been seeing the following error while trying to send emails to some domains:

Final-Recipient: name@example.com
Original-Recipient: name@example.com
Action: failed
Status: 5.0.0
Remote-MTA: dns; mail.example.com

Diagnostic-Code: smtp; 550 "The mail server detected your message as spam and
    has prevented delivery."

Neither my IP address nor my domain is blacklisted. The email is a simple test email without attachments or links.

What can I do or where can I get more information about this issue?

Thank you