OptionalbodyRaw message bytes, when the body was fetched.
IMAP flags set on the message (e.g. \Seen, \Flagged).
OptionalheadersParsed RFC 822 headers, lowercased keys, when the body was fetched.
OptionalinternalServer-side internal date string, when present.
OptionalsizeOctet size reported by RFC822.SIZE, when requested.
Unique identifier within the selected mailbox.
A single message returned by ImapSession.fetch.