The advantages of HTML emails
For the inexperienced HTML seems the obvious choice. HTML is much more visually appealing, creates a stronger sense of brand identity and improves legability. Why then would anybody consider using plain text?
The problems with HTML emails
Unfortunately the apparent advantages of HTML emails are undermined by a considerable number of problems many of which revolve around SPAM:
As I am sure you are all too aware SPAM has become a major problem. In fact it is estimated that 70% of all emails travelling the web at anyone time are SPAM. As a result many users choose to place stringent restrictions that block anything that might be considered SPAM. Unfortunately that often means blocking HTML messages even when those come from a legitimate source such as an opt-in mailing list.
Another problem with HTML emails is that they have a significantly higher chance of carrying a virus than plain text. This has been forcing email software manufacturers and corporations to place stronger restrictions on the receiving of HTML emails. In fact many modern email clients such as AOL 9.0 and Outlook 2003 have actually blocked HTML emails from downloading any external content or containing many attachments. This means that the majority of HTML emails do not display properly in these clients.
One of the major selling points of HTML email, as I have already explained, is that they are visually more attractive. However it is phenominally hard to be sure exactly how your email will display when it reaches its destination. Many email clients display HTML emails incorrectly while others strip out some of the HTML code and replace it with its own code.
HTML emails also take much longer to download than plain text and so there are bandwidth considerations. This can lead to users deleting the email before viewing it out of pure frustration.
There are still many email clients that simply cannot interpret HTML emails. The result is that they show the code of the email as plain text. This means that it is often impossible to read the email between all the HTML code.
Another concern for many users, especially the more technically lierate ones, is that HTML emails can contain code that intrudes privacy. For example an HTML email can contain code that tells the sender that the email has been received and read. This means the SPAMMER knows they have sent to a valid email address. This is another reason why so many modern email clients block HTML emails by default.
For the 73% of us still using dial up connections HTML email can cause another annoying problem. When you use dial up it is normal practice to connect to the internet, download your email and then disconnect before reading them. The problem with HTML emails is that they often draw content directly from the web. This means that when you view an HTML email it will automatically reconnect to the internet without the users permission. Even if the user has changed his settings to prevent this from happening it still causes to the email to display incorrectly.
My conclusion is simple. In a world of SPAM and viruses, use plain text. More and more email is seen as intrusive and annoying. If you need the impact of HTML then the best approach will be to keep your plain text email short and punchy. Encourage the user to click through to a web page that can add the visual impact you are looking for.