I frequently send emails that have to be formatted carefully (the newsletters, for example). I have found it easy to get carried away in constructing a good-looking layout using the HTML editor built-in to my emailer software; fonts, colours - the works. What I had forgotten to remember is that some of my recipients will have opted to receive email in plain text format, rather than HTML format.
Consider a recent example in my own mailbox. This is the email viewed in HTML format:
Although I have set my email software to block the loading of images (a useful safety feature), it looks good - much as the originator wanted it to look. But if my email settings were set to "plain text", then the same email would look like this:
Not easy to read at all and it doesn't give a good impression.
Every time you send a email with a lot of formatting (font, colours), always check it viewed as plain text before sending.