If you spend much time on the marketing web you've likely seen plenty of hot takes on the topic of plain text emails, with often wild claims about them being more effective than HTML email from companies with a vested interest in getting you to use their product. Cough, cough.
The reality is, of course, not so simple as some may claim, particularly when you start talking about an email that's sent to more than one person.
GoSquared wrote a fantastic breakdown of the Uncanny Valley effect they'd observed in some of the emails they received. It's well worth a read, not only as a counterpoint to the plain text email brigade but in improving your email marketing in general.
Can plain text emails work? Absolutely. But avoid the obvious pitfalls and don't unintentionally mislead people when using them.