Applies read-state, label, and contact changes to a message in a conversation. Omitted fields are left unchanged. The read flag is only valid on received messages. Label changes to apply. Labels in `add` are applied and labels in `remove` are taken off; other labels are left untouched. Adding a label that is already present, or removing one that is not, has no effect. System labels express state changes: on a conversation, adding `spam` files it as spam, adding `archive` files it away without deleting it, adding `inbox` (or removing `spam` or `archive`) returns it to the inbox, and removing `unread` marks all retained received messages as read in one call; on a message, adding or removing `unread` flips read state, and adding or removing `trash` moves it to or out of the trash. Changes that contradict this model are rejected: adding more than one placement label in one request, adding `blocked` (blocking a sender is a receive-rule decision), removing `inbox` without adding a destination, adding `trash` or `unread` to a conversation (removing `unread` is the mark-all-read shortcut; `trash` uses the DELETE verb), placement labels on a message (move its conversation instead), and `unread` on a sent message. Custom labels are 1-64 characters with no commas, control characters, or leading or trailing whitespace. System label names and a small reserved set (`all`, `archived`, `deleted`, `draft`, `drafts`, `flagged`, `important`, `junk`, `muted`, `none`, `outbox`, `pinned`, `read`, `scheduled`, `snoozed`, `starred`) cannot be used as custom labels, in any casing. A conversation or message carries at most 20 labels, system labels included. Labels to apply. Labels to take off. Contact to link this message to, or null to unlink the current contact. Message ID. Received messages carry a `rem_` ID, sent messages an `em_` ID — the same IDs used by the received-message and sent-message logs. Direction of the message — `inbound` for a received message, `outbound` for a sent one. Possible values: `inbound`, `outbound` Channel this message was carried on. Always `email`. Conversation this message belongs to. Sender address. Recipient addresses on the To line. Recipient addresses on the Cc line. Empty when the message had none. Address the message was actually delivered to, when it differs from the mailbox address (for example mail routed in from another address). Null for sent messages and for mail addressed directly to the mailbox. Message subject. Null when the message had no subject. Short plain-text preview of the message body. Plain-text content of the message with quoted history stripped — readable for the mailbox's full retention period, both directions. Always present when fetching a single message; on list endpoints it is included only when the request sets `include=extracted_text`. Null when no text could be extracted. Labels on this message. System labels carry its state: a received message holds exactly one placement label — `inbox` for accepted mail, `archive` when its conversation was filed away, `spam` (failed sender authentication), or `blocked` (rejected by the mailbox's receive policy or rules) — plus `unread` until it is read. `trash` marks a message in the trash, either direction. Custom labels share the same list; a message carries at most 20. Folded delivery status of a sent message: `accepted`, `sent` (provider handoff), `delivered` (all attempted recipients delivered), or `failed` (terminal failure). Null for received messages. Terminal per-recipient delivery outcomes of a sent message, folded in as they become known — part of the message's durable memory. Null for received messages and before any recipient reaches a terminal state. Per-recipient event detail lives on the sent-message log (`source`) for 30 days. Whether the sender of a received message was authenticated. `pass` means the sender's identity was verified; `fail` means it was checked and did not verify; `unknown` means no verdict could be determined and the sender should not be treated as verified. Null for sent messages. Part of the message's durable memory — readable for the mailbox's full retention period, so the verdict survives after the 30-day inbound log has expired. Possible values: `pass`, `fail`, `unknown`, `null` Whether SPF passed for the sender of a received message. Null for sent messages and when no verdict is available. Durable for the mailbox's retention period. Whether DKIM passed for the sender of a received message. Null for sent messages and when no verdict is available. Durable for the mailbox's retention period. Whether DMARC passed for the sender of a received message. Null for sent messages and when no verdict is available. Durable for the mailbox's retention period. When the message will be permanently deleted: the end of the mailbox's retention period, pulled nearer (at most 30 days out) while the message is in the trash. Restore a trashed message before then with `PATCH {"labels": {"remove": ["trash"]}}`. Number of attachments on the message. Attachment metadata (filename, content type, size). Remains readable for the mailbox's retention period even after the attachment bytes themselves have expired. Attachment ID, used to download the attachment bytes. Original filename, or null when the attachment had none. MIME content type, or null when it could not be determined. Attachment size in bytes. RFC 5322 References header entries used to thread the conversation. Contact linked to this message, or null when none is linked. Link to the message's entry in the received-message or sent-message log, which carries delivery analytics such as per-recipient events. Log entries expire 30 days after the message occurred. API path of the log entry for this message. When the log entry (and the message's original rendered source) expires. When the message was received or accepted for sending.