I've set "send as" permission for a user to a distribution group, however, after 4 days, outlook is still throwing errors of "You can't send a message on behalf of this user unless you have permission to do so". I've done the below.
In ADu&c, in the security tab of the distribution group, I've added the user and checked the "send as" checkbox.
I am running Exchange 2010 sp1, the distribution group is a universal distribution group. I've tried this on OWA and I get a red error saying something similar to the NDR I get from outlook.