I just added a confirmation dialog when changing your email address.I think the confirmation dialog should test the address. That is, send a confirmation message to the address and display the SMTP result - success or failure. I'd go further and keep the Yes button disabled until there is a success code.
The intent is to head off the confusion that occurs when the member makes a typo in the address.
There are some complications, of course. One is the round-trip time for the SMTP response. So perhaps there should be an activity indicator showing that Groups.io is testing the address. Maybe there should be a third button that the user can use to confirm the address despite the lack of response (maybe even in the face of a failure response).
I'm not exactly sure why a user would ever click the "confirm anyway" button. Perhaps they know the address isn't ready for use yet but will be, or perhaps they know the address engages in greylisting or other spam-control protocol that interferes with the test. Of course, to me that would be a red-flag but I'm not going to rule out the possibility that the user actually does know what they're doing.
And also there is the false-positive problem: it /has/ happened to me that a typo in a Direct Add address was a good address belonging to someone else entirely. One could go the further step of requiring that the user verify receipt of the test message. Perhaps using a confirmation code in that message to by typed into a box in this dialog. I'm not sure I want to force that level of functionality at this point, but it may not be an unreasonable requirement and would head off the typo that is someone else's address case.