Full-text search query matched against subject, body, and addresses. Supports web-search syntax: bare words are AND-ed (invoice overdue), "quoted phrases" match in order, -word excludes, and or alternates (invoice or receipt). Words are stemmed, so approving matches approval. Results are ranked by relevance, with subject matches weighted above body matches above address matches. For meaning-based rather than keyword matching, use POST /messages/search/semantic instead.