Size

• Email messages composed using HTML can be significantly larger than plain text emails, as they contain numerous formatting codes, color codes and other style information that is not present in plain text emails. This means that low-bandwidth users will have a much faster response rate viewing plain text emails. They also tend to have images, which makes them even larger.
  
  

Security

• Plain text emails are simply displayed and not interpreted by the email client. They are also unable to contain embedded images or resources that are loaded as images off the Internet. HTML emails open the possibility of JavaScript exploits, browser history theft, logging of personal information such as the IP address, location and time of opening of the message and other creative applications of Web technologies that cannot be employed using plain text.
  
  

Content

• HTML emails have many more stylization options and can contain embedded movies and images. It is not possible to include any style on a plain text email.
  
  

Deliverability

• The use of HTML in an email increases its score in popular anti-spam applications, such as spam assassin. Furthermore, HTML code is often examined by the mail server before being delivered and may be flagged as a risk by the server's administrator.