From The Writer's Digest:
"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)".
https://www.writersdigest.com/write-better-fiction/what-is-the-plural-form-of-email-grammar-rules
I agree with that as, indeed, does the New York Times:
https://www.nytimes.com/2010/05/09/magazine/09FOB-onlanguage-t.html
. . . and countless other sources too!
Further, even if 'emails' didn't exist as a plural, it would still exist as the present tense of the verb 'to email'. (e.g. "He emails yet another complaint to his boss nearly every day").