The arm package-repositories need some work… they are lagging behind the x86-64 repo and the centos repo’s.
However openLdap and mail is supported but there is one cave-cat:
Scanning for Viruses with clam-av (nethserver-filter) demands a lot of memory and if you are planing to use full mail filter support on a Rasberry pi you will find out it is a bit to demanding…
and therefor it is optional for arm.