HTML emails are harder to read, larger, take longer to download, facilitate all kinds of viruses and security risks, are impolite, are mostly spam and deleted by some spam traps and can't be read by all email clients. Be nice: Send plain text emails.
Millions of people do not have a client that can read HTML emails. Your email comes out as a very large blob of unsightly text, characters and is mostly unreadable. Some universities, companies, individuals and offices only have plain text email clients, for some of the reasons listed below. Sending HTML emails is impolite and presumptious.
It is best to turn off HTML emails on your system, so that your machine will not automatically open them, and just display them in plain text. HTML emails allow companies to insert images that act as trackers for your email address. Also, all email viruses and email security breaches use the scripts that can be run inside HTML emails. Without HTML email, there would have been none of the email viruses that have plagued users, they simple couldn't spread.
"Most HTML messages are at least twice as big as
plain text only, and they can be many time larger." (From George Dillion.com)
"HTML emails are more than twice the size of plain text emails. [...] Even with a broadband connection, such wasted bandwidth adds up over thousands of messages per week. It slows down my email client (more crap to parse through) and wastes my time." (By Marc Zeedar)
They connect to the internet by itself (From George Dillion.com)
"If you're off-line, opening an HTML email cantaining images may (by default) open a connection to the internet."
On my machine, I have told my email client to use precisely the font and font size that I find best. So, plain text emails are very easy to read as they are displayed perfectly customized the way I want them. HTML emails, however, set their own fonts and sizes and are unsightly and more difficult to read. Also, many who send HTML emails use standard HTML behavior including paragraph tags that do not create an empty line, etc, making it even worse.
Companies who send large quantities of emails always send it in HTML email, because they can insert images that track the email as soon as it is opened. The images are retrieved from their server, they simply add an ID number to the URL of each image, and therefore track your email address. HTML email is equated with SPAM, making it immediately irritating, annoying and impolite. Many programs that try to reduce spam will automatically flag or delete HTML emails. If you are coversing with someone and they're using HTML emails, it is very awkward to have to go take their emails out of quarantine each time they send any.
People other than spammers send HTML emails for various reasons. Those who are new to the Internet, the ignorant, the impolite and the young are all frequent HTML email senders because they in general don't realize the pitfalls.
"The main people who use HTML email are those who don't know how to turn off that option in their email client, and the marketers, who want to sell. For those who don't know, here are great instructions on how to turn off HTML in a bunch of email clients."