I'm talking here about the word, not the thing itself. Originally, mail sent electronically was known as e-mail. This form of the word has been steadily losing ground to email. I'm here to say that this is wrong.
I've read all of the arguments in favor of dropping the hyphen, and none of them are convincing. Usually, the hyphen-less form isn't so much argued for, as assumed, with a mumbled justification of "words change over time." Which indeed they do, but not every change is for the better.
The common argument for dropping the hyphen is that the hyphen is originally used to signal the connection between two words that have not been commonly seen together to that point. After sufficient time passes, the hyphen is no longer needed. Thus, to the use the Strunk & White example, bed chamber becomes bed-chamber becomes bedchamber.
Here's the problem, though. In other cases, dropping the hyphen is a way of simplifying the language, making the word look more like it is pronounced. That is not the situation with e-mail.
Why? Because in e-mail, the e is literally the letter e. It stands for the word electronic, of course, but it's not a shortened form of the word electronic, which starts with a short-e sound. To see the distinction, suppose that mail sent electronically had been originally called electro-mail. Dropping the hyphen in this case would cause no problem, as electro-mail and electromail would be pronounced the same. But email looks like a word that is pronounced uh-MAIL, not EE-mail.
An analogy would be G-Man, a slang term for an FBI agent. The G is literally the letter G, standing for government, but not a shortened form of the word government. Therefore it would look ridiculous to write Gman or Gmen.
Likewise, writing email for e-mail complicates rather than simplifies. Some would argue that everybody knows how to pronounce email already, so what difference does it make how it is spelled? But that road leads to madness. Why not spell it eml, or 3ma!1 or whatever kooky string of letters pops into our head?