Let's face it. We are always looking for efficient and quick ways to assemble email marketing campaigns. One of the most common shortcuts is the copy & paste function (good 'ol "ctrl-c" then "ctrl-v"). However, if you create subject lines for campaigns in your email marketing software by copying and pasting content from programs like MS Word, you may end up distributing emails that display strange characters in the subject line when viewed by recipients. Why does this happen and what can you do about it?
As explained in this enlightening post from the Email Deliverability blog, the technical explanation for such gaffes is that many email applications only support the ASCII character set when rendering subject lines. However, MS Word and other word processing software employ characters that are non-ASCII (e.g., slanted double quote, apostrophe/single quote, em-dashes), and thus incapable of being rendered properly by recipients' email clients.
Thus, to make sure that your subject lines display properly across the greatest number of email clients, you should only use ASCII character sets in your subject lines. Which means typing your subject lines, rather than copying and pasting content from your word processing application, or even web pages.
For the intrepid, to learn more about the difference between ASCII and non-ASCII character sets,and encoding, you can download this short white paper from Pivotal Veracity.