Before we get started on building or configuring Sendmail, let's take a look at some of the things that you need to be concerned with. There are some main areas where you need to make decisions. These are inbound message routing, outbound message routing, system preference settings, disk layout and backups. Backups will not be covered here. Any decent backup strategy can be used with Sendmail.

The things that have an affect on incoming message are:

Is the Sendmail MTA daemon running?
Do you have aliases?
Are the DNS MX records setup properly?
Does class w match the mx records so mail doesn't bounce?
Are you going to check for unsolicited commercial e-mail?
How are you going to setup the hard disks?
Which mailer will deliver the message, mail.local, procmail, others?
What options have you selected, maximum message size, etc.?
Are you using SMTP Authentication or Transport Layer Security?
Do you have enough disk space?
Are your databases setup correctly?
Is your mail filtering setup correctly?

All of these things will have an affect on your ability to receive messages. What you'll mainly need to focus on are which machines will receive the incoming message, and then what is the final destination of that message. After the message has reached its final destination then how will the end user read that message?

When it comes to the hard disks, are you going to NFS mount the server's disks back to the workstations or are you going to install POP3 or IMAP on that machine so users can read their mail? If you're not going to NFS mount the hard disks back to the workstation or provide POP3 or IMAP services, are the users just going to logon to the host machine and use Sendmail or some other client to read their messages? If you're not sure how to NFS mount the disks, check into purchasing a copy of "NFS and NIS" second edition at OReilly.com. It will walk you through the different configuration options available and show you how to setup your machines to handle this topic. In addition, http://docs.sun.com under Solaris 10, System Administrator, Network Services, Mail Services Topics can also assist you with this endeavor.

At the end of the day, what steps have you taken to ensure that if the machine you're using for mail services decides to take a vacation then what are your plans to get up and running again?

After the end user has finished reading their messages, what happens when they actually decide to respond to an e-mail message or they just decide to fire off a message to someone else? That's when you need to be concerned with outgoing mail. What affects outbound e-mail?

Are you doing any relaying?
Are you doing any masquerading?
Is your /etc/nsswitch.conf file configured properly?
Is your DNS Server configured properly?
Is your DNS Client configured properly?
Is your /etc/hosts file setup properly?
Is your firewall blocking access to the other site?

The machine that initially receives the e-mail message from the outside does not have to be the machine that handles final disposition of that message. It could simple forward it on to some other host. The same is true of outbound e-mail.

If you're running firewalls you may not want all of your internal hosts machines to send e-mail directly to the outside world. You may want all of your hosts to send their Internet bound messages to a few hosts, and allow only those hosts to be able to send messages directly to the Internet.

There are a lot of things that you can configure in Sendmail. You can set limits on things such as maximum message size, you can set file permissions, you can set privacy options, and a whole slew of timeouts in the sendmail.cf file.

All the different features of Sendmail and all of the timeout settings that you can set will be listed as well as a brief description of each item.

I'll also discuss some tuning options and configuration changes that you may want to take advantage of to help Sendmail run more smoothly.

The heart and soul of Sendmail are the rewriting rules. These rules take up a good chunk of the sendmail.cf file. These rules allow Sendmail to handle different situations. These sections are extremely terse. The reason it's terse is the faster that Sendmail can parse it; the faster it can do its job. If you have a good grasp of regular expressions then the ruleset sections will be easy for you to decipher.

Not everything that happens in Sendmail is handled by the sendmail.cf file. Sendmail also uses external databases to help it perform its function. These databases can hold aliases and mailing lists. You can also setup address rewriting using these databases as well as setting a catch all mailbox for your domain.

The thing to keep in mind is you really don't need to tweak Sendmail unless you want certain things to happen that aren't already happening. The default installation will work and do things in a generic fashion for you. You only need to modify the files if you want things to work differently then they already are.

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