Problem: Emails from your site go out in a good looking HTML format, but some of the content you want to inject in those emails are plain text while others are already formatted with HTML. The email content of the plain text emails which are only plain text now appear as one big block of text with no line breaks.
Let's look at this problem specifically in the context of a Drupal website.
All the emails such as welcome, password recover, etc... are plain text by default. Now you want to add a nice email template utilizing a module like mimemail.