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 because the address couldn't be found. Check for typos or unnecessary spaces and try again. LEARN MORE
eventual consistency much?