get group from ad allow them to send email to one distribution groups

Hello All,

I want allow emails from external contacts listed in my active directory to members of particular departments in many physical locations. I have OUs in based on physical location and every location has that department lets say IT department. AD has external contacts which are will be added to a new group and that group will be allowed to send emails to IT department in all the physical locations.

To summarize, 

I plan to 

1. Create security group and add those external contacts to that group.

2. Set the permissions on the existing distribution groups in exchange to receive emails from newly created AD security group.

the issue here is the number of Distribution groups are huge is there some power shell script which can help me out in achieving this goal.

Thanks, :)

MYM