New fields on POST /v1/send:cc, bcc (a string or a list each) and replyTo. cc recipients are named in the Cc header; bcc recipients receive the message but are named in no header; replyTo is written as Reply-To.
Rolling out per Motor Block. A Motor Block that has not been moved to the new sending path answers 422 with code: "feature_not_enabled" and sends nothing, so a copy is never silently dropped.
One list of rules for everyone.to, cc and bcc together may name at most 50 addresses; an address is used once; every recipient is checked against the sandbox and Free-account rules; a suppressed recipient is skipped and reported in data.skipped.