If you test an email address using our Mail Server Test Center and you receive a result of "Mismatch" for the DNS test, this means that your reverse DNS entry does not match your forward DNS entry.
For example, in order for domain.com to pass the dns mismatch test with mail.domain.com as its MX record, the reverse for the IP for the MX record would need to point to the MX host mail.example.com.
*NOTE* - Please keep in mind that if you recently made a change to your DNS records to correct this issue, you may need to wait for propagation in order to see the correct result when re-testing.