spiegelman:

Long emails are, more frequently than not, the worst. When you send someone an email, you make a demand on their time. If you use more words than necessary, you waste their time. Sure we’re talking maybe a fraction of a minute, but given the number of emails the average person sends in a day those fractions add up pretty quick.

This conflicts with an older style of correspondence that associated pleasantries with tact. Tactful emails now are efficient, and pleasantries are a waste. People accustomed to pleasantries see their absence as rude, or a sign of being cross. They infer a tone that isn’t there, while people accustomed to brevity know how difficult it can be to ascertain tone from an email.