Distribution Groups

Not sure if this is an Exchange question or something related to AD.

In AD I have the following:

BranchManager <- OU

  Branch1 <- Universal security group

  Branch1 <- Universal security group

In Exchange I want to setup a distribution group that sends an email to all of the people in Branch1 and Branch2. I can create a distribution list that uses either Branch1 or Branch2 but not both.

I tried to create a universal security group that contains both Branch1 and Branch2 but the distribution list didn't send mail to it.

Does anyone have any ideas on how I can create a distribution group that works, in this case for multiple branches?

Thanks