There is a feature called Cross Forest Delegation introduced with Exchange 2007. It enables a user in forest A to manage a calendar of a user in forest B.
This feature is supported by ILM FP1 SP1 and FIM 2010 - prerequisites are
- a GALsync
- Forest Trust between Forests
- Cross-Forest Availability
A Recipient Type of Cross-forest mail contact is created and you can use this contact to assign delegate permissions.
You may create this type of contact manually - some attributes need to have certain values, described in MCS UK Unified Communications Blog.
It seems that FIM is not a quick to install software. So many administrators are looking for an easy to understand, easy to install, easy to use product like NETsec GALsync - to reduce complexity.
NETsec GALsync targets Exchange organizations which have NO trust and are totally independent. Cross Forest Delegation requires a trust - so it is out scope in this software. Nevertheless GALsync supports to sync all attributes which you need to realize a Cross Forest Delegation.