There are many subtleties involved in electronic mail. These
include:
Email is (usually) accepted for delivery on an SMTP
relay host, usually located in the same organisation as
the sender. Such a system is sometimes also called an SMTP
gateway. In the event that the destination system is not
available (eg, is down or unreachable), the relay host "spools" the
message, and attempts to deliver it at regular intervals. The
emergence of spam email in recent
years has resulted, in part, from improperly-configured email
relays.
A email address usually defines a mailbox, not
a person (but see below). A mailbox is simply a
text file, in mailbox format, on the destination host. It is not
necessarily true that a destination mailbox resides on the actual
host to which a message was sent...
There are other forms of email address other
than mailboxes. These include:
mail forwarders
mail aliases
mailing lists
automated mail systems, ie the destination is a process.