Poll – PbEM or PBeM?

Lately I’ve noticed that I’ve been writing “PbEM” instead of “PBeM”. Making the “b” lower case because it’s the insignificant word “by”. But I’ve noticed that when other people use the acronym, they make the “e” small, for “e-mail”. I’m not quite sure when this convention came about, and why certain letters deserve to be uppercase more than others. But I wondered enough about it that I wanted to create a poll to see which was the most popular.