What is a Blacklisted Email Domain?

It means that your email's domain or IP address has been has been identified as spam.

How Does an Email Domain Get Blacklisted?

There are a variety of reasons why an email can be blacklisted, some possibilities are: the email address was spoofed (your email was forged by another sender), you have sent too many outbound emails, you've received too many complaints by those receiving your emails, and in some circumstances the account has been hacked.