No rule for this one
No ending or word group predicts the article of E-Mail. Learn it as a pair and say it out loud: die E-Mail.
die E-Mail
It's die E-Mail. E-Mail is feminine, plural die E-Mails.
Ich schreibe eine E-Mail.
I write an email.
No ending or word group predicts the article of E-Mail. Learn it as a pair and say it out loud: die E-Mail.