Email Relay Server:
A security risk in an e-mail server allows an external user to relay mail to other mail servers. Spammers abuse open relays to hide the source of their messages.
Inves DNS or Reverse DNS:
When the system is sent to a DNS IP address and domain name is returned. Reverse DNS is used to log incoming traffic by domain name for statistical purposes. It is also used to prevent spam by determining if the email from the domain name indicated in the message header. Reverse DNS is only an option and is not required on a DNS server.
Reverse DNS is a popular method for catching spammers who use invalid IP addresses. If a spam filter or program can not match the IP address for the domain name, which can reject the email.
Creativity, which includes video, animation or sound. Rich e-mail messages through media often collect high open rates and click, but require more bandwidth and are less compatible with different email clients regular text or e-mail messages in HTML format.