Per writer's digest:

Email is the plural of email unless specific "emails" are discussed:

Email is the plural form of email, much like mail is the plural form of mail, when discussing email in a general sense. As in, I've got a lot of email. Very general, very correct, but...

Emails is the plural form of email when you get into specific numbers. As in, I've received 10 emails from him this morning. This is unlike the plural form of mail, since you wouldn't say you received 10 mails since last week, though you might say you've received 10 letters (though probably more likely bills or advertisements).