The next step in the process is to determine if you should use plain text or HTML text. First, realize who your customers are. Are they likely to have an email reader that allows for HTML? Today, most people do have this. And, since most do, you should be using it.
Plain text emails are something some people like, though. For many opt in lists, you can ask which type of email they would like. Your autoresponder or email program will handle sending the individual the proper type of email for them at that point. You can check out the self hosted autoresponder at https://myselfhostedsetup.com
There are benefits to using either of these options in your email marketing campaign. For example, when you use plain text, the email opens and is able to be read, giving you more of a chance to be read when compared to a long loading HTML message.
On the other hand, HTML messages do give you many more options in terms of design. You can craft a highly attractive type of email. This may be more professional looking. If you do use HTML messages, be sure you do not over do it in the graphics department. No one will wait long to see what your "this is taking forever" email has to offer. They will close it and move on without thinking twice about it.