also: メールとも
noun
email friend; email penpal
Refers to a friend or acquaintance with whom one communicates primarily via email, often without having met in person. Common in early internet culture.
彼女はメル友と毎日メールをやり取りしている。
She exchanges emails with her email friend every day.
メル友に会ったことはないけど、もう何年も連絡を取り合っている。
I've never met my email penpal, but we've been in touch for years.
Abbreviation of メール友 (mēru tomo), from メール (email) + 友 (friend).