Generally, you will compose mail using a mailer interface like mail or mailx, or more sophisticated ones like mutt, tkrat, or pine. These programs are called mail user agents, or 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, or 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. On UUCP systems it is not uncommon to see mail delivery handled by two separate programs: rmail for remote mail delivery and lmail for local mail delivery.
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 bounced, that 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.
Mail is usually not delivered directly in UUCP networks, but rather is forwarded to the destination host by a number of intermediate systems. To send a message over a UUCP link, the sending MTA usually executes rmail on the forwarding system using uux, and feeds it the message on standard input.
Since uux is invoked for each message separately, it may produce a considerable workload on a major mail hub, as well as clutter the UUCP spool queues with hundreds of small files taking up a disproportionate amount of disk space. Some MTAs therefore allow you to collect several messages for a remote system in a single batch file. The batch file contains the SMTP commands that the local host would normally issue if a direct SMTP connection were used. This is called BSMTP, or batched SMTP. The batch is then fed to the rsmtp or bsmtp program on the remote system, which processes the input almost as if a normal SMTP connection has occurred.
This is because disk space is usually allocated in blocks of 1,024 bytes. So even a message of a few dozen bytes will eat a full kilobyte.