I created a new secondary account for some reason, and tried to send ping-pong emails from both accounts.
1. secondary to primary works!
2. replying to secondary's mail from primary works!
3. create new email -> primary to secondary fails spectacularly with following error.
Address not found
Your message wasn't delivered to <email address> because the address couldn't be found. Check for typos or unnecessary spaces and try again.
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eventual consistency much?
5. retry: create new email -> primary to secondary email. Fail!