Local delivery of mail is, of course, more than just appending the incoming message to the recipient's mailbox. Usually, the local MTA will understand 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. If the mail must be delivered over a network using TCP/IP, SMTP is commonly used. SMTP stands for Simple Mail Transfer Protocol, and is defined in RFC-788 and RFC-821. SMTP usually connects to the recipient's machine directly, negotiating the message transfer with the remote side's SMTP daemon.
In UUCP networks, mail will usually not be delivered directly, but rather be forwarded to the destination host by a number of intermediate systems. To send a message over a UUCP link, the sending MTA will usually execute rmail on the forwarding system using uux, and feed it the message on standard input.
Since this is done for each message separately, it may produce a
considerable work load on a major mail hub, as well as clutter the UUCP
spool queues with hundreds of small files taking up an unproportional
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 was
used. This is called BSMTP, or batched SMTP. The batch is then
fed to the rsmtp or bsmtp program on the remote system,
which will process the input as if a normal SMTP connection had
occurred.