It would be very helpful to have the ability to customize which domains are allowed to subscribe to a subscription list. We would ideally like to target specific email domains allowed to subscribe to a list. If you do not match a specific @domain.com address then you get a message that states you are not authorized to subscribe to this list. The idea is ultimately wrapped around a customer who has made it clear that subscription links are being forwarded outside of our control, thereby allowing anyone to join a list. As you can understand this poses a significant security concern.