Thank you for submitting this feedback. The reason the email address is unable to be edited is because it's Confirmed, which means they were in Awaiting Confirmation and confirmed their subscription. Given that they confirmed with that specific email address, it becomes locked for editing. We do have a small note on this article about not being able to edit an address if it's confirmed, but I agree it is hidden.
I've passed the feedback along to the engineers and we will take this into consideration when working on our Contacts system, hopefully making it more clear in the account itself. Until then I have opened this post for voting. Thank you again!
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