For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:buschmann23:MTA-STS/openSUSE_Tumbleweed/home:buschmann23:MTA-STS.repo
zypper refresh
zypper install postfix-mta-sts-resolver
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:buschmann23:MTA-STS/15.5/home:buschmann23:MTA-STS.repo
zypper refresh
zypper install postfix-mta-sts-resolver
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:buschmann23:MTA-STS/15.4/home:buschmann23:MTA-STS.repo
zypper refresh
zypper install postfix-mta-sts-resolver