Still have not come around to installing this and configuring it for the current project.. would like to get back to this. At this stage, only preliminary research has been done for the email configuration with Rails. A longer, more in depth post will come at a later point in time.
Configure OpenSMTPD as Relay
https://wiki.archlinux.org/index.php/OpenSMTPD#Relay_only
https://www.opensmtpd.org/announces/release-6.4.0.txt
https://coderwall.com/p/eejzja/simple-smtp-server-with-opensmtpd
https://xw.is/wiki/OpenSMTPD_forward_to_Google
https://man.openbsd.org/smtpd.8
Make sure OpenSMTPD is working with sendmail
https://man.openbsd.org/sendmail.8
Configure rails to properly use Sendmail
https://guides.rubyonrails.org/action_mailer_basics.html#example-action-mailer-configuration
http://openbsd-archive.7691.n7.nabble.com/OpenSMTPD-and-Rails-What-to-do-with-i-and-t-td230589.html
Configure Rails in a container to properly use Sendmail
-v /usr/sbin/sendmail:/usr/sbin/sendmail
Test Rails Development
Test Rails Production
Docker OpenSMTPD Container Image
https://github.com/wodby/opensmtpd