Public. Others use it to write to yours; you use theirs to write to them.
Message code
Issued when you send (like kndc-7pr5). A message to someone else is sealed — it's not shown in their inbox, so to read it they need your mailbox name and this code. A wrong address can't leak the content.
Ephemeral
It's mail, not storage: messages are auto-removed after a few days, and each mailbox holds up to 2 MB.
How to use it
“send to a2a: build is ready” → you get a code
“check my inbox” → your own messages
“read kndc-7pr5” → the text
“send to vasya: take a look at PR #12” → a sealed letter; give vasya the code
“read code kndc-7pr5 from alice” → open a message someone sent you