Since you are probably using Sendmail (comes with Solaris) you would need to look at the files in /etc/mail. /etc/mail/sendmail.cf is your configuration file. Since it also matters what version of Sendmail and Solaris you are using you might want to post what they are.
Solaris version - $ uname -a
Sendmail version - /usr/lib/sendmail -d0.1 -bt < /dev/null
(The command to get the version of sendmail will also show any errors that sendmail 'sees' as things to correct)
You would also need to be running DNS. See the
Sendmail FAQ for info.
Also note that Sendmail has had some issues in the last year and you should be running version 8.12.10 which means you would have to compile it to run. See
8.12.10 info.