Originally Posted by Susie Smith

Email is short for electronic mail. So it's a kind of mail. Mail is considered uncountable, isn't it?

I received many pieces of mail. / I received much mail.
I received many e-mail messages. I sent a lot of e-mail.

I'd say that e-mail is uncountable but that it's "acceptable in informal English" to say e-mails because a lot of people do it.