Sendmail on Solaris by default is configured to run in secure mode.
Therefore, it only listens on the local loopback interface, ie, 127.0.0.1.

In order to enable it to listen on its network interface, the following command must be executed:

svccfg -s sendmail setprop config/local_only = false

which then enables sendmail to listen on the configured network interface listed in sendmail.cf under /etc/mail.