In case you didn’t know, OpenSMTPd is so outdated on Ubuntu systems, that you’ll need to install it from sources, otherwise expect some TLS issues 🙂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.