OptionalattachmentsFiles rows to attach, resolved to bytes host-side. The host bounds these at 10 attachments and 25 MB of total source bytes per message and refuses past either — it never truncates the set.
OptionalbccOptionalccOptionalhtmlOptionalreplyReply to a message seen through messages.read on the same account. The
host resolves its Message-ID/References headers and thread id; the
caller never writes a mail header.
OptionalsubjectOptionaltextOptionalto
Structured compose input; the host builds the RFC 2822 payload from it and never accepts caller-supplied raw MIME.