A delegate allowed to send as a shared mailbox (or a locked account handed to it) could do so only through JMAP; over SMTP the sender check knew only the login's own addresses and masks, so a service signed in as a person got 501 5.5.4 for the mailbox's address. - MAIL FROM also accepts an address whose owner the login holds a current delegation with sendAs into (exact address, no catch-all). - MA-S3 over SMTP: a send as a shared mailbox is refused at DATA when a From or Reply-To address isn't the mailbox's own, as JMAP refuses it. - MA-D0a / AL-9: a queued message sent as another account is audited (QueuedMessage target, "Sent as X, from <login>"); a locked account's delegate gets the AL-9 record. SMTP sends as a group, unrecorded before, are recorded the same way. - The SMTP session keeps the signed-in AccessToken for this. Tests: account_lock_tests' shared mailbox part sends as support@ over SMTP (delivered, audited), is refused a foreign Reply-To, and a desk without sendAs still gets 501.