For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:Kolab:16.2/15.6/server:Kolab:16.2.repo
zypper refresh
zypper install cyrus-imapd
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:Kolab:16.2/15.5/server:Kolab:16.2.repo
zypper refresh
zypper install cyrus-imapd
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:Kolab:16.2/15.4/server:Kolab:16.2.repo
zypper refresh
zypper install cyrus-imapd