Email and Online mail

Email noun - (computer science) a system of world-wide electronic communication in which a computer user can compose a message at one terminal that can be regenerated at the recipient's terminal when the recipient logs in.

Online mail and email are semantically related in sending topic. In some cases you can use "Online mail" instead a noun "Email", when it comes to topics like electronic mail.

Email and online mail are semantically related in sending topic. You can use "Email" instead a noun phrase "Online mail", if it concerns topics such as electronic mail.