Whether a copy of every message sent from mcom(1) is appended to the account's sent mailbox, passed as himalaya message send --save sent.
sent is resolved through the account's [mailbox.alias] map, which the module fills in from . An account that has no sent mailbox is an error rather than a message silently sent without a copy.
Turn this off when the server files its own copy, as a JMAP submission does.