If I receive an auto-response from a subscribers email stating that they have a new email address (new address given in body of respons), can I legally add their new email address to my distribution list?
This is kind of a gray area as far as Compliance. On the strict side of precaution the answer would be no, since each individual email address is considered unique in its permissions and what's acceptable to send stuff to. With that said, more often than not someone updating their email address to a new one and providing an auto-response with the new address is a pretty good indication that they're okay receiving emails from you - especially if the old address already had a significant amount of communication / rapport established.
In this regard, you may wish to reach out to the contact's new address, just to confirm they're okay receiving your emails at that one. You're also welcome to reach out to our Review/Compliance teams to see what specialized insight they have on the matter.