Foundations
Message format, headers, MIME, identity, and the envelope.
Required
Internet message format
Every message must use the standard header and body structure, valid line endings, and legal field syntax before it enters SMTP.
RequiredRequired message header fields
Each message needs a valid Date, From, and globally unique Message-ID, with recipient and subject fields that match its purpose.
RequiredMIME multipart structure
Use valid MIME types, boundaries, transfer encodings, and multipart nesting so every client can select a safe representation.