In case you didn’t know, OpenSMTPd is so outdated on Ubuntu systems, that you’ll need to install it from sources, otherwise expect some TLS issues 
You will need to use the following:
./configure \
--with-user-smtpd=opensmtpd \
--with-user-queue=opensmtpq \
--with-group-queue=opensmtpq
mkdir -p /var/empty
ln -s /etc/ssl/certs/ca-certificates.crt /usr/lib/ssl/cert.pem
Congrats, now you have a proper working SMTP server.
Cheers.