Generally, you will compose mail using a program such as mail or mailx, or more sophisticated ones such as mutt, tkrat, or pine. These programs are called mail user agents (MUAs). If you send a mail message, the interface program will in most cases hand it to another program for delivery. This is called the mail transport agent (MTA). On most systems the same MTA is used for both local and remote delivery and is usually invoked as /usr/sbin/sendmail, or on non-FSSTND compliant systems as /usr/lib/sendmail.
Local delivery of mail is, of course, more than just appending the incoming message to the recipient's mailbox. Usually, the local MTA understands aliasing (setting up local recipient addresses pointing to other addresses) and forwarding (redirecting a user's mail to some other destination). Also, messages that cannot be delivered must usually be bouncedthat is, returned to the sender along with some error message.
For remote delivery, the transport software used depends on the nature of the link. Mail delivered over a network using TCP/IP commonly uses Simple Mail Transfer Protocol (SMTP), which is described in RFC 821. SMTP was designed to deliver mail directly to a recipient's machine, negotiating the message transfer with the remote side's SMTP daemon. Today it is common practice for organizations to establish special hosts that accept all mail for recipients in the organization and for that host to manage appropriate delivery to the intended recipient.