Для openSUSE Tumbleweed запустите от имени root:
zypper addrepo https://download.opensuse.org/repositories/home:gladiac:mailman/openSUSE_Tumbleweed/home:gladiac:mailman.repo
zypper refresh
zypper install mailman3
Для 16.0 запустите от имени root:
zypper addrepo https://download.opensuse.org/repositories/home:gladiac:mailman/16.0/home:gladiac:mailman.repo
zypper refresh
zypper install mailman3
Для 15.6 запустите от имени root:
zypper addrepo https://download.opensuse.org/repositories/home:gladiac:mailman/15.6/home:gladiac:mailman.repo
zypper refresh
zypper install mailman3