Just a few notes to add:
After adding the repos you need to run:
signal-event yum-modify
before:
yum --enablerepo=nethsme install smeserver-sogo
Otherwise yum will complain about no repo found "nethsme"
And from SME Server 8b5 you don't need to do:
yum update e-smith-ldap --enablerepo=smeupdates-testing
Interesting contrib otherwise, thanks.