Mail addressed to one host can be routed to another host for a variety of reasons: forwarding, relaying, mailertable entries, and so on. One common reason for routing mail in this manner is because DNS says to do so. A system sending mail routes the mail based on information obtained from DNS. Mail is addressed to some hostname. The remote system takes that hostname and asks DNS if it has mail exchange (MX) records for that host. If no MX record is found for a given host, the address record of the host is obtained from DNS, and the mail is sent directly to the host. If DNS returns MX records, the remote system sends the mail to the system with the lowest preference number listed on the MX record.

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Add smmsp to /etc/passwd

When sendmail is run as non-set-user-id root, it is run either as root when it is invoked by the root user, or as another user when it should not run as root. The sendmail distribution clearly cannot divine ahead of time what user you wish to use when not running sendmail as root. It could have chosen nobody, for example, but the user nobody does not exist under all versions of Unix.

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There are two approaches to installing a new sendmail:

•  If you choose to run the new sendmail in place of the original, you first need to create and install a new configuration file. The m4(1) program is used to automate the process of configuration file creation.
• If you choose to keep the original and install the new sendmail in parallel (until you can test it), you can proceed with the installation and defer configuration files until later. Note that this choice presumes you customized the file locations.

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The first step in compiling sendmail is to establishan object directory and a Makefile that is appropriate to your machine architecture and operating system. You do this by running the Build script in the sendmail source directory:

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