Installing and Administering Internet Services

Chapter 4 139
Installing and Administering sendmail
How sendmail Works
If your host has a direct UUCP connection to the next host in the path,
the mail is delivered to that host through UUCP. If not, the message is
returned with an error. The supplied configuration file provides detailed
instructions for arranging to relay such mail through hosts to which you
can connect.
SMTP Addresses. RFC 822-style addresses in any of the following
forms, where host is not the local host name, are routed by SMTP over
TCP/IP:
user@host
[e-mail]
<@host,@host2,@host3:user@host4>
user@[remote_host’s_internet_address]
If the name server is in use, sendmail requests MX (mail exchanger)
records for the remote host. If there are any, it attempts to deliver the
mail to each of them, in preference order, until delivery succeeds.
Otherwise, sendmail connects directly to the recipient host and delivers
the message.
Mixed Addresses. The supplied configuration file interprets address
operators with the following precedence:
@, !, %
This means that recipient addresses using mixtures of these operators
are resolved as shown in Table 4-2.
Table 4-2 How sendmail Resolves Addresses with Mixed Operators
MX Records
The BIND name server, if it is in use on your host, provides MX (Mail
Exchanger) records. These can be used to inform sendmail that mail for
a particular host can be relayed by another host, if the addressed host is
Address Mailer Host User Recipient
user%hostA@hostB TCP hostB user%hostA@hostB user@hostA
user!hostA@hostB TCP hostB hostA!user@hostB hostA!user
hostA!user%hostB UUCP hostA user@hostB user@hostB