Sometimes you might want to have Sendmail masquerade as a host other than the actual hostname of your system. Such a situation could occur if you have a dialup connection to the Internet and your ISP handles all your mail for you. In this case, you want Sendmail to masquerade as the domain name of your ISP. For example:
MASQUERADE_AS(`samplenet.org')dnl
Using Smart Hosts
If you do not have a full-time connection to the Internet, you probably want to have Sendmail send your messages to your ISP's mail server and let it handle delivery for you. Without a full-time Internet connection, you could find it difficult to deliver messages to some locations (such as some underdeveloped areas of the world where email services are unreliable and sporadic). In those situations, you can configure Sendmail to function as a smart host by passing email on to another sender rather than attempting to deliver the email directly. You can use a line such as the following in the sendmail.mc file to enable a smart host:
define(`SMART_HOST', `smtp.samplenet.org')
This line causes Sendmail to pass any mail it receives to the server smtp.samplenet.org rather than attempt to deliver it directly. Smart hosting will not work for you if your ISP, like many others, blocks any mail relaying. Some ISPs block relaying because it is frequently used to disseminate spam.
Setting Message Delivery Intervals
As mentioned earlier, Sendmail typically attempts to deliver messages as soon as it receives them, and again at regular intervals after that. If you have only periodic connections to the Internet, as with a dialup connection, you likely would prefer that Sendmail hold all messages in the queue and attempt to deliver them at specific time intervals or at your prompt. You can configure Sendmail to do so by adding the following line to sendmail.mc:
define(`confDELIVERY_MODE', `d')dnl
This line causes Sendmail to attempt mail delivery only at regularly scheduled queue processing intervals (by default, somewhere between 20 and 30 minutes).
However, this delay time might not be sufficient if you are offline for longer periods of time. In those situations, you can invoke Sendmail with no queue processing time. For example, by default, Sendmail might start with the following command:
# sendmail -bd -q30m
This tells Sendmail that it should process the mail queue (and attempt message delivery) every 30 minutes. You can change 30 to any other number to change the delivery interval. If you want Sendmail to wait for a specific prompt before processing the queue, you can invoke Sendmail with no queue time, like this:
# sendmail -bd -q
This command tells Sendmail to process the queue once when it is started, and again only when you manually direct it to do so. To manually tell Sendmail to process the queue, you can use a command like the following:
# sendmail -q
If you use networking over a modem, there is a configuration file for pppd called ppp.linkup, which is located in /etc/ppp. Any commands in this file are automatically run each time the PPP daemon is started. You can add the line sendmail -q to this file to have your mail queue automatically processed each time you dial up your Internet connection.
Building the sendmail.cf File
Books are available to explore the depths of Sendmail configuration, but the Sendmail Installation and Operation Guide (check on Google for this) is the canonical reference. Configuration guidance can also be found through a Google search; many people use Sendmail in many different configurations. Fortunately, Fedora has provided a default Sendmail configuration that works out of the box for a home user as long as your networking is correctly configured and you do not require an ISP-like Sendmail configuration.
After you have made all your changes to sendmail.mc, you have to rebuild the sendmail.cf file. First, back up your old file:
# cp /etc/mail/sendmail.cf /etc/mail/sendmail.cf.old
You must run sendmail.mc through the m4 macro processor to generate a useable configuration file. A command, such as the following, is used to do this:
# m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
This command loads the cf.m4 macro file from /usr/share/sendmail- cf/m4/cf.m4 and then uses it to process the sendmail.mc file. The output, normally sent to STDOUT, is then redirected to the file sendmail.cf, and your new configuration file is ready. You have to restart Sendmail before the changes take effect.
Fedora also provides an alternative to using awk to rebuild the Sendmail configuration. As root, execute the following:
# make -C /etc/mail
Mail Relaying
By default, Sendmail does not relay mail that did not originate from the local domain. This means that if a Sendmail installation running at hudson.org receives mail intended for hudzilla.com, and that mail did not originate from hudson.org, the mail is rejected and not relayed. If you want to allow selected domains to relay through you, add an entry for the domain to the file /etc/mail/relay-domains. If the file does not exist, create it in your favorite text editor and add a line containing the name of the domain that you want to allow to relay through you. Sendmail has to be restarted for this change to take effect.
You need a very good reason to relay mail; otherwise, do not do it. Allowing all domains to relay through you makes you a magnet for spammers who want to use your mail server to send spam. This could lead to your site being blacklisted by many other sites, which then will not accept any mail from you or your site's users — even if the mail is legitimate!
