By default sendmail is installed, though in "local" mode.
So the matter is how to "open" sendmail and configure it as a SMTP server.
$ pkg info sendmail
Summary: Sendmail utilities
Description: SMTP-based mail transfer agent sendmail(1M), its configuration
hierarchy sendmail(4), the corresponding local delivery agent
mail.local(1M), and various other mail-related utilities.
Build Release: 5.11
Packaging Date: March 22, 2013 09:28:10 PM
Size: 3.07 MB