The Collaborative International Dictionary of English v.0.48:

Electronic mail \E`lec*tron"ic mail\, n. (Computers)
   1. a message transmitted from one computer to another,
      accessible by means of a mail reading program on the
      receiving computer. The message may have one or many
      intended recipients, and may be directed by the sending
      program to one or to multiple receiving computers. The
      message is typically in the form of a computer file, and
      may be a simple ASCII text, or any other type of binary
      coded information

   Syn: email. [PJC]

WordNet (r) 3.0 (2006):

electronic mail
    n 1: (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; "you
         cannot send packages by electronic mail" [syn: electronic
         mail, e-mail, email] [ant: snail mail]

The Free On-line Dictionary of Computing (30 December 2018):

electronic mail
e-mail

    (e-mail) Messages automatically passed from one
   computer user to another, often through computer networks
   and/or via modems over telephone lines.

   A message, especially one following the common RFC 822
   standard, begins with several lines of headers, followed
   by a blank line, and the body of the message.  Most e-mail
   systems now support the MIME standard which allows the
   message body to contain "attachments" of different kinds
   rather than just one block of plain ASCII text.  It is
   conventional for the body to end with a signature.

   Headers give the name and electronic mail address of the
   sender and recipient(s), the time and date when it was sent
   and a subject.  There are many other headers which may get
   added by different message handling systems during delivery.

   The message is "composed" by the sender, usually using a
   special program - a "Mail User Agent" (MUA).  It is then
   passed to some kind of "Message Transfer Agent" (MTA) - a
   program which is responsible for either delivering the message
   locally or passing it to another MTA, often on another host.
   MTAs on different hosts on a network often communicate using
   SMTP.  The message is eventually delivered to the
   recipient's mailbox - normally a file on his computer - from
   where he can read it using a mail reading program (which may
   or may not be the same MUA as used by the sender).

   Contrast snail-mail, paper-net, voice-net.

   The form "email" is also common, but is less suggestive of the
   correct pronunciation and derivation than "e-mail".  The word
   is used as a noun for the concept ("Isn't e-mail great?", "Are
   you on e-mail?"), a collection of (unread) messages ("I spent
   all night reading my e-mail"), and as a verb meaning "to send
   (something in) an e-mail message" ("I'll e-mail you (my
   report)").  The use of "an e-mail" as a count noun for an
   e-mail message, and plural "e-mails", is now (2000) also well
   established despite the fact that "mail" is definitely a mass
   noun.

   Oddly enough, the word "emailed" is actually listed in the
   Oxford English Dictionary.  It means "embossed (with a raised
   pattern) or arranged in a net work".  A use from 1480 is
   given.  The word is derived from French "emmailleure",
   network.  Also, "email" is German for enamel.

   The story of the first e-mail message
   (http://pretext.com/mar98/features/story2.htm).

   How data travels around the world
   (http://www.akita.co.uk/movement-of-data)

   (2014-10-07)