Because, for testing purposes, I keep deleting and regenerating IDs I have old IDs in the contacts list of different nodes. I noticed when sending to an older ID, from a regenerated ID, that when trying to respond to the message, Mail was trying to send to the old nickname from the contact list, which was now invalid, rather than to the correct address for the new incoming ID with the same nickname.

My suggestion is that Mail should at least warn the user when the incoming ID has the same nickname, but is not the same address as, the existing address linked to that nickname contact. It could perhaps also offer an option to replace the entry in the contact list.