Why you shouldn't use HTML email...
_ ASCII ribbon campaign ( ) against HTML email X / \
It can provide statistics to spammer and marketers by opening remote images on their server (called html email web bugs) and identify you with cookies. Spammers use HTML emails to confirm your address when sending out their junk blindly.
Sending HTML email wastes a lot of bandwidth compared to plain text, with duplicate plain text / html versions in a single email, and wastes precious storage space on smaller devices such as PDAs and cellphones.
It is not a standard and is not supported in some email readers (this is what HTML email looks like). Many companies support department also only accept plain text with their queue system.
It has obfuscated code that makes it difficult to archive and search, thus reducing its usability. It also doubles the content (text and html), making searches result more complicated.
It can initiate an internet connection if it contains remote elements such as images. You don't want Outlook in your wireless notebook dialing on its own at $4/minute in Singapore while you aren't paying attention.
HTML emails are slower to render for older computers and other slower devices such as cell phones, text consoles or PDAs.
The majority of people trying to create html email will choose color combinations that are horrific to the reader and give them headaches after a long day at work. Let's face it, only a minority has real graphic design talents, but many more think they do and victimize us ;-)
The majority of people sending html formatted email don't know enough about graphic file formats and optimal file sizes and can create huge email with graphics of multiple megabytes, especially if they use high speed internet (I have seen background graphics of up to 4 megabytes which also rendered the email totally unreadable. I was laughing but the recipient wasn't.).
HTML is mostly used by spammers and marketers and is often automatically filtered in the trash, especially with the recent versions of Thunderbird or Outlook equipped with Bayesian filtering, so you risk having your message discarded as well.
Sending html email forces a specific appearance of the text that might go against the reader's own personal preference in his settings (font size, colors, etc). Also, you may choose a font that is not present on the user system (different platforms, etc) and the email will not appear as desired. Forwarding the message may also alter its appearance and make it unreadable.
HTML is refused, and impossible to manage, in most mailing list and Usenet newsgroups.
Quoting is clearer and easier with plain text, and most e-mail readers can detect the standard > symbol and interpret it as they like (Thunderbird makes nice vertical ligns). Tables in HTML can make quoting impossible.
People who read their e-mail with alternative devices, like cell phone, text consoles in terminals, PDA's, etc, will have great problems if the width of your email is fixed in HTML, or contain images, etc.
Spammers can disrupt the HTML code with comments like "Casi<!-- test -->no", so that filters trying to pick up on the "Casino" keyword will not detect it. Better just filter HTML formatted emails in the trash altogether.
by using more complex layouts and presentational effects, it wastes ink when printing it, which is not very environmental friendly
The HTML code generated by your email client can be faulty or non standard, and not readable even with other readers that support HTML. Web based email services like hotmail or yahoo! can also have great difficulties in displaying a badly written HTML email (I've seen some mails disfigure the whole Yahoo Mail interface).
With plain text, what you see and read is what the email file is made of, there is no hidden attachment or multi-parts to the message. Also, you don't run a risk of inserting personal information like Microsoft Word does in Word documents (computer name, address, etc), which I'm sure many html email softwares do.
Plain text is easier to read for people with disabilities and special text-to-speech programs.
Other Links on the use of HTML email
Plain text email has been around for years with a proven record of security, it is consistent and always reliable, and is the de facto standard.