Forwarding a forwarded forward
Usually reserved for mothers, grandmothers, and other novice emailers, the forwarded forwarded forward usually contains some sort of animated .gif and Comic Sans font.
Check out the rest and vote for the worst offender after the break.
Unnecessarily replying all
Though often employed as a joke, this tactic usually just annoys everyone on the email chain — especially if it involves some sort of fun plan that some recipients may not be able to participate in.
Purposely sending a (no subject) email
I am so guilty of this. It's not that I don't have the time to add in a subject line, it's that I don't want to have to sum up an entire email into just a few words. But it makes searching for emails far more difficult, and it's easier for it to get overlooked.
Reckless use of the ! indicator
This might be my biggest pet peeve. With exceptions like emails from your boss and exceptionally pressing matters, adding the ! to an email that is probably not !-worthy makes me cringe. I will prioritize! I promise!