For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:frispete:mail/openSUSE_Tumbleweed/home:frispete:mail.repo
zypper refresh
zypper install postfix
For openSUSE Leap 15.2 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:frispete:mail/openSUSE_Leap_15.2/home:frispete:mail.repo
zypper refresh
zypper install postfix
For openSUSE Leap 15.1 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:frispete:mail/openSUSE_Leap_15.1/home:frispete:mail.repo
zypper refresh
zypper install postfix