Для openSUSE Slowroll запустите от имени root:
zypper addrepo https://download.opensuse.org/repositories/server:php:applications/openSUSE_Slowroll/server:php:applications.repo
zypper refresh
zypper install squirrelmail-plugins
Для openSUSE Factory запустите от имени root:
zypper addrepo https://download.opensuse.org/repositories/server:php:applications/openSUSE_Factory/server:php:applications.repo
zypper refresh
zypper install squirrelmail-plugins
Для devel languages php tumbleweed запустите от имени root:
zypper addrepo https://download.opensuse.org/repositories/server:php:applications/devel_languages_php_tumbleweed/server:php:applications.repo
zypper refresh
zypper install squirrelmail-plugins
Для 15.6 запустите от имени root:
zypper addrepo https://download.opensuse.org/repositories/server:php:applications/15.6/server:php:applications.repo
zypper refresh
zypper install squirrelmail-plugins
Для 15.5 запустите от имени root:
zypper addrepo https://download.opensuse.org/repositories/server:php:applications/15.5/server:php:applications.repo
zypper refresh
zypper install squirrelmail-plugins