For openSUSE Leap 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:mail/openSUSE_Leap_15.6/server:mail.repo
zypper refresh
zypper install kelbt
For SLE 15 SP2 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:mail/SLE_15_SP2/server:mail.repo
zypper refresh
zypper install kelbt
For SLE 15 SP1 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:mail/SLE_15_SP1/server:mail.repo
zypper refresh
zypper install kelbt
For SLE 12 SP5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:mail/SLE_12_SP5/server:mail.repo
zypper refresh
zypper install kelbt