Other Aspects

• 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.
   
  
• 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.