For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mbozicevic/openSUSE_Tumbleweed/home:mbozicevic.repo
zypper refresh
zypper install OpenSMTPD
For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mbozicevic/16.0/home:mbozicevic.repo
zypper refresh
zypper install OpenSMTPD
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mbozicevic/15.6/home:mbozicevic.repo
zypper refresh
zypper install OpenSMTPD