Add or remove the same labels on 1 to 50 messages. At least one of add_labels or remove_labels is required (400 missing_labels). Empty message_ids is 400 missing_message_ids. Duplicates are 400 duplicate_message_ids. More than 50 ids is 400 too_many_ids. read and unread can change. Other system labels return protected_label, invalid_label, or label_conflict. Ids that are not in this inbox are omitted. Found messages all update, or none do. The response is a list of full messages in request order, with limit set to the request length and count set to how many updated. Works on a paused inbox. A held message keeps the held label and a withheld body. An inbox-scoped key for another inbox is 404 inbox_not_found.