Current information about open source sendmail after the acquisition of Sendmail, Inc. by Proofpoint, Inc. in 2013 is available on the Proofpoint website. Information for earlier versions of Sendmail up to 8.15.2 (released in 2015) is available on this website. Also, an archive of Sendmail, Inc. Documentation, Tips and Tricks, and Support FAQ files is available on this website.

An introduction to setting up the sendmail daemon versions 8.10.0 and later on many Linux systems to support Internet Protocol version 6 (IPv6) appears below. Because of the many variations among implementations and between versions of Sendmail, differences are to be expected. Earlier versions can be more involved to setup, when it is even possible.

1. Make sure you have completed the applicable steps described in this Get Your Site Ready for IPv6 article for the host computer your Sendmail daemon is installed on. (That article was written for a web server, but the DNS-specific steps also apply to Sendmail.)
2. Sendmail by default typically adds a Daemon_Options for port 587. When you want that behavior, omit the first line below. Otherwise, include the first line and sendmail will then use just port 25.

To configure sendmail to listen on both IPv4 and IPv6 at the same time:

FEATURE(`no_default_msa', `dnl') dnl
DAEMON_OPTIONS(`Port=submission, M=Ea, Name=MSA, Family=inet6') dnl
DAEMON_OPTIONS(`Port=smtp,Name=MTA, Family=inet6') dnl

Then "make" and restart sendmail.

On many O/Ses, sendmail will accept IPv4 via the IPv6 socket without any explicit IPv4 configuration. If not, it may be necessary to use the line:

DAEMON_OPTIONS(`Name=MTA-v4, Family=inet, Name=MTA-v6, Family=inet6') dnl

instead of the last line above.

A more extensive example of an IPv6 configuration for sendmail is available here.

For FreeBSD, an example configuration file is available here.