When you attempt to delete a distribution group or security group from the ECP (Exchange Control Panel) of Exchange 2013, the following error may occur:
You don’t have sufficient permissions. This operation can only be performed by a manager of the group.
There are 2 ways to fix this:
1. In an Exchange Management Console, execute the following cmdlet:
Remove-DistributionGroup <NameOfGroup> -BypassSecurityGroupManagerCheck
2. In Active Directory, change the Managed By person in tab. You can then click Change… and enter your admin (ECP) account. After that, return to your ECP and delete the distribution group.
By the way, you can also view the ManagedBy attribute in Active Directory. When you have enabled Advanced Options in dsa.msc, you can open the Attribute Editor of the specific distribution group and note the account specified as value in managedBy.