I had this question after viewing Setting Send As Permissions on a Mail Distribution Group
I have the same issue as above and I've seen the proposed solution before, but I'm not sure if/how it applies here.
In my situation, I just migrated from 2007 to 2013. It appears any groups that have existed prior to being migrated (and upgraded) to 2013 now prevent me from being able to manage send-as rights. I get the insufficient permissions error as indicated in the link above.
With the EAC I created a brand new DL in the same OU as the groups I cannot manage. I have zero issues managing this group, including its send-as rights.
The test above would indicate that the Exchange Trusted Subsystem only has issues with the pre-existing DL, but I exported the AD permissions of a pre-existing group with the issue and my new test group. The permissions are 100% identical.
I don't understand what's going on here. Further I don't understand the statement that Exchange Trusted Subsystem by default does not have permissions to manage send-as. This makes no sense to me as why would MS give you ability to manage the groups via EAC/Powershell but not assign the necessary permissions to do so?
At any rate, I'm looking forward to any suggestions as to how to remedy my issue as well as explain why the previous solution was even proposed and why it's necessary (when it seems not to be necessary for newly created groups).