Whether you hate or love HTML email, chances are good that at least one of the messages in your inbox today was written in it. There are lots of reasons to write HTML email and as many reasons to avoid HTML email.
Email is for communication.
The point of email is to communicate. And the better your message comes across, the more likely people will read it and benefit from it. Styled emails make it easier to see the message (usually) because elements that are important can be highlighted and lesser elements downplayed.
But styles can't always be seen. I received a message reporting a typo in one of my documents, and the sender highlighted the typo in red. Because I read my email as text, the red did not show through and I spent even more time trying to find the typo than I would if the sender had simply stated "you wrote 'there' instead of 'their' in the third sentence."
Testing Email is virtually impossible.
If you're like many Web designers, you despair at the thought of testing a significant portion of the Web browsers out there. Email is 10 times worse. Email is a much older system than the Web, and as such has many more clients set up to use it. Many corporate clients (such as Lotus Notes) can only be tested in a coprorate network using that system.
Even if the client supports HTML, some people still read it in text mode.
You might consider these people to be extremely technophobic, but many people prefer to read email in text mode rather than HTML and most email clients allow them to turn off HTML.
HTML emails are more likely to be flagged as spam.
SpamAssassin and other spam blockers often use the amount of HTML in an email message as an indication of whether the email is spam. This is because spammers discovered HTML email much quicker than the average email user, and HTML is much more effective in generating a click than plain text.
Styled email gets better results.
Styled email, with images and fancy text, can be more pleasant to read than plain text. So people reading your email messages are more likely to respond to the message or take action because of the message.
HTML email can be a security risk.
HTML email enables malicious senders to embed trojans and other malicious entities into an email message. Some email clients will automatically open attachments and images embedded in HTML email, and this can cause a system to be infected without the victim taking any action other than downloading email.
HTML email is longer to download.
Email is slightly different than Web pages, in that most people don't wait for one email message to download. But it can be very frustrating for your recipients if a message is so big that it stalls all the rest of their mail. And HTML email, is, by definition, larger than the same message in just text. Because there are extra tags surrounding the text that is displayed. Plus, images, whether they are attached or linked, need to be loaded and that can increase the time it takes for the email to load.
HTML email is fun to write.
Just like writing Web pages, HTML email is fun to write. You can spend time actually designing how your message will come across, rather than relying on just the text to convey your feelings. If you've ever been criticized for an email message you sent where someone thought you were angry when you were just excited, HTML email can help with that - because you can add images and colors to convey your meaning, where you would have had to use ALL CAPS or ** and // characters to emphasize and de-emphasize your points.
HTML email can generate privacy issues.
Some companies use 1x1 pixel images (called webbugs) to track who opens the email message. While most companies don't track customers on an individual basis, some people still feel this is a violation of their privacy. They would like the opportunity to refuse to be tracked, and webbugs don't offer that. If you read the HTML message, you've viewed the image and the fact that the email was opened is recorded.
HTML email is fun to read.
Email that arrives with images and style is a lot more fun to read than an email that is just text. This is especially true for designers and other visually oriented people. Text is not always the best way to get a point across - as the saying goes "a picture is worth 1000 words." And an embedded image inside an email is much more likely to be seen and appreciated than one that's sent as an attachment.
Many email clients don't reliably support CSS.
This is mostly an issue for designers who are learning to write standards based HTML for their Web pages. When they turn around to put those same designs in their email messages, many clients choke. CSS is still not well supported in email clients, except for very basic styles like bold, italics, and colors. Layout in HTML emails should be done with tables.
HTML email makes phishing scams much easier.
One of the primary ways that phishing scams work is to imitate a legitimate company's email format and change the links to point to the scammer's site. While it's possible to see that the links don't go where they appear to go, most people don't look and assume that if it "looks like my bank, it must be my bank." Hopefully, this isn't as true any longer.
HTML emails make it much easier to send non-English email.
If you need to send email in a non-ASCII compliant language such as Russian, Korean, or Hebrew, you need to use Unicode or some other language encoding. While you can use RTF, HTML is more standardized and more email clients on more platforms understand it and so can render the message correctly.
Many email clients default to HTML email.
Clients like Outlook Express and AOL use HTML email by default to provide their users with text formatting and embedded images. It can be very difficult to turn off HTML email in these clients.
Some people really hate HTML email.
There are some people out there who are highly offended by the use of HTML email and will delete these messages unread.
Some people really love HTML email.
On the other hand, some people love HTML email. They find it much easier to read and prefer it to plain text.