Fedora Linux

7.6.1.6. Configuring Masquerading

sendmail includes masquerading capability, which enables outbound mail to be modified so that it looks like it came from another system. This is commonly used to remove hostname information from the email address. To configure bluesky.fedorabook.com so that outbound mail appears to be from user @fedorabook.com instead of user @bluesky.fedorabook.com , locate the MASQUERADE_AS line in /etc/mail/sendmail.mc :

dnl # The following example makes mail from this host and any additional
dnl # specified domains appear to be sent from mydomain.com
dnl #
dnl MASQUERADE_AS(Qmydomain.com')dnl

Uncomment the MASQUERADE_AS line and replace mydomain.com with the domain name you wish to use:

MASQUERADE_AS(Q fedorabook.com ')dnl

7.6.2. How Does It Work?

Fedora's email system, like most others, is divided into three parts:

mail transport agent (MTA)

Transports mail between systems. sendmail is the default MTA.

mail delivery agent (MDA)

Delivers mail to local users, optionally performing filtering or sending vacation replies ('Jane is away from the office until Monday; she will read and reply to your mail when she returns'). Fedora uses procmail in this role.

mail user agent (MUA)

The email client that interacts with the user. A Fedora user can choose from many different MDAs, including Evolution, Thunderbird, SquirrelMail, and the text-based mail command.

Originally written when a wide range of email transportation schemes were in use, sendmail is designed to route mail through and between these different systems, each with their own address format and message queuing system. Because of this heritage, sendmail has a sophisticated and complex configuration system, but many of the configuration options are not used for Internet email servers.

sendmail is now used almost exclusively with the Simple Mail Transport Protocol (SMTP), which is a human-readable transfer protocol that uses TCP/IP connections on port 25. You can use telnet to connect to an SMTP server and manually send mail if you want:

$ telnet concord2.proximity.on.ca smtp
Trying 127.0.0.1...
Connected to concord2.proximity.on.ca (127.0.0.1).
Escape character is '^]'.
220 concord2.proximity.on.ca ESMTP Sendmail 8.13.5/8.13.5; Thu, 2 Mar 2006 13:07:11 -0500
EHLO fedorabook.com
250- concord2.proximity.on.ca Hello concord8.proximity.on.ca [127.0.0.1], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE
250-DSN
250-ETRN
250-AUTH DIGEST-MD5 CRAM-MD5
250-DELIVERBY
250 HELP
MAIL From: [email protected]
250 2.1.0 [email protected]... Sender ok
RCPT To: [email protected]
250 2.1.5 [email protected]... Recipient ok
DATA
354 Enter mail, end with '.' on a line by itself
Subject: Greetings!Date: Thu, Mar 2006 12:08:11 - 0500
Hi there -- just dropping you a quick note viatelnet. Hope your day is going well.
-Chris .
250 2.0.0 k22I7BTo016133 Message accepted for delivery
QUIT
221 2.0.0 concord2.proximity.on.ca closing connection

Notice the blank line separating the email headers from the message bodyjust like HTTP transfers. The HTTP format is derived from the email format.

You can also send mail by sending it to the standard input of a sendmail process:

$ /usr/bin/sendmail [email protected]: Test II
Did you remember to renew the domain registration?If not, please take care of this before next Tuesday.[Ctrl-D]

Outbound mail is queued in /var/spool/mqueue /. Inbound mail is delivered via procmail to users' mailboxes in /var/spool/mail/ . The mailboxes are simply text files containing all of the messages concatenated end to end; this format is sometimes called mbox format .

The /etc/mail/sendmail.mc file used for configuration is an m4 macro file. It is interpreted by the m4 command using files in /usr/share/sendmail-cf/m4/ to build /etc/mail/sendmail.cf .