These days we get a lot of emails that are HTML rendered, which allows the sender to color the fonts, style the text as per their will and also allows them to embed images. This makes the emails very attractive and frankly, it does make it more easier to read the mail, after all that’s what HTML was built for, (Hyper Text Mark-up Language 
).
But had you ever wondered that behind all these glitz and glamor of the colored fonts, images and styled texts, a security threat always lurks around… unbelievable right ? Read on to figure how spammers try to make use of this glitz and glitter to take over your computer to make it their host…
It starts with all those HTML rendered emails hiding the specially formed links or scripts, hidden anchor words or image, which track your every action performed within the mail.
Now you might be wondering how one profits from figuring out if you read the mail or not, but there are commercial companies that make these software to track email, providing a means for the sender to know whether an email was read, when it was read, and even follow its tracks if it is forwarded to others. Well this software may not be famous amongst every regular user, but for spammers are trying to figure whether an email address found randomly on net is valid, or whether the recipient has a tendency to open spam, sounds like a perfect gift :). Once they study the behavior of the user, they send more such similar mails and make their intentions quite obvious, which is to SPAM!Plain text email does not support embedded images. Plain text email squashes web bugs.
Some of the other common problems a user might face because of these HTML-rendered mails are that these mails can cause the email attachments to open automatically, or even forcefully download files on to the system. One such famous example is that Winevar virus, contained active content in its email that first modified the System Registry to specify .CEO files as executable, and then automatically opened the attached - and infected - .CEO file it had smuggled past the scanners. This was done in order to bypass content filters that prohibited EXE files in email.
Plain text email does not support active content. Plain text email prevents email attachments from opening automatically.
And the most common of all, phishing scams. Do a Google search for “paypal scams” and you’ll find tons of results and carefully investigating you’ll learn that these scammy mails are a hidden format of these HTML-rendered emails, where in there are cases that folks have given information about their bank details or credit card details!! Scary, isn’t it ?
Plain text email provides a true WYSIWYG (What You See Is What You Get) experience. In plain text email, there are no hidden commands - the link displayed is the actual link.
So all in all, if you get HTML rendered emails, then be cautious enough not to fall prey to all the scammers out there.