Speaking of email sins, I have a developing pet peeve: off-topic emails in a chain of group emails.
Fr'instance: I email with my band a lot. There are 7 of us, and we have a lot going on. We'll have an email thread about, say, a date check for an upcoming gig. Inevitably, somebody will stray OT, and not only does that bug me, it also makes one thread unnecessarily long, when it could be two (or three) easy-to-find and easy-to-follow individual threads. Worst is when they're one line responses.
Ooh, that's another one! A one line response to a long email chain, and the sender doesn't bother to clip the 12 emails below it. Granted, Gmail does that for me, but still...
Or the folks with a 14 line Notice of Privacy at the end of an email that says "Yep." Grrr...
Okay, I have a lot of email pet peeves.