After installing Linux Mint, I found that I did not have an option on the Evolution Preferences -> HTML Messages tab, to make sure that all email I received was only displayed as plain text. The option display was as per Figure 1 below.
If you too do not see the Plain Text Mode -> Show suppressed HTML parts as attachments and a HTML Mode drop down as in Figure 2, exit the Evolution email client, then open a terminal window and install the evolution-plugins package as follows
sudo apt-get install evolution-plugins <enter>
This installs the standard evolution plugins and one of them will enable the HTML Mode selection drop down.