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Thursday, August 28, 2008

sendmail

Email services on the Virtual Private Server are managed by the sendmail program.

sendmail is a configurable, robust SMTP server that processes all incoming and outgoing messages for all User Accounts with email permissions. In order to check or send email from a remote client (such as Outlook or Eudora) the user must have POP or IMAP permissions.

Configuring sendmail

The sendmail program runs as a daemon (a program that is always running). For sendmail to run properly there should be two daemons as well as the SASL authentication daemon running. When you make changes to any of the sendmail configurations you must restart sendmail to load the new settings. To restart sendmail, you must be the root user. Connect to your Virtual Private Server via SSH, su to root, and run the following command:

# restart_sendmail

You can also change to the /etc/mail directory and run the following command:

# make restart

Start and stop can also be used with the command such as:

# make restart stop

# make restart start

check_sendmail is new tool that will print the status of the two sendmail daemons and the SASL authentication (saslauthd) daemon.

NOTE: For more information on the SASL authentication daemon see the man page:
# man saslauthd

check_sendmail will restart any of these daemons if they are not running. To run the check_sendmail tool run the following command:

# check_sendmail

You can easily use the Cron program schedular to schedule check_sendmail to regularly run and check the status of the sendmail daemons. Add an entry similar to the following to your crontab:

*/30 * * * * /usr/local/sbin/check_sendmail

This will run the command /usr/local/sbin/check_sendmail every thirty minutes.

Domains, aliases, and forwards

When an email message arrives, sendmail checks the incoming domain and determines if it is either a local domain or an authorized relay domain. In addition to the VPS hostname (or primary domain), which is always considered a local domain, you can have any number of local domains on your server. In order to be considered local you must have the domain listed in the /etc/mail/local-host-names file on your server. Once a domain is determined to be local, sendmail checks the virtusertable and then the aliases to determine if there are any special delivery conditions for the recipient address.

Once an email recipient has been checked against the virtusertable for domain based delivery, the domain name of the recipient is ignored by sendmail. The username, without the domain, is then checked against the aliases, and finally delivered to the correct local user's mailbox.

The VPS v2/v3 can also be configured to relay email messages for authorized domains. This allows you to use the VPS v2/v3 as a secondary mail server. When an email message arrives with a recipient domain that is listed in the /etc/mail/relay-domains file, sendmail will try to deliver the message to the primary server. If the primary server is unable to accept the mail, sendmail will hold the messages in a special queue, and will deliver them to the primary mail server once the primary server is online again. This feature is especially useful to companies using exchange servers, or who have limited network access to their primary mail server.

Mail Filtering and SPAM prevention

In addition to recipient based email routing, sendmail can be configured to filter or route messages based on other elements of the message. The easiest way to do this is with Procmail, an easy to configure mail filtering add-on. You can also configure sendmail itself for filtering using the M4 macro set.

While there are many possible reasons to filter mail, one of the most common uses is to prevent SPAM (or UCE)

Other Popular Email Tools

There are a number of additional email tools that you can configure for the VPS v2/v3. Some of the more popular ones include Email Mailing List Managers, Email Encryption Tools, and Email Autoreply.

Using Remote Mail Clients

For VPS v2/v3 users with Mail permissions, POP and IMAP compatible remote mail clients can be used to download and send email messages. For more information, see Email Clients.

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