for Authentication (obsoletes , updates , updated by ) – Email Submission Operations: Access and Accountability Requirements (BCP 134) – A Registry for SMTP Enhanced Mail System Status Codes (BCP 138) (updates ) – The Simple Mail Transfer Protocol (obsoletes aka STD 10, , , , updates ) – Internet Message Format (obsoletes aka STD 11, and ) – Downgrading Mechanism for Email Address Internationalization – Message Submission for Mail (STD 72) (obsoletes , ) – The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages (obsoletes , and in turn ) – SMTP Extension for Internationalized Email Addresses (updates , , , and ) – Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access See also Bounce address CRAM-MD5 (a SASL mechanism for ESMTPA) Email Email encryption DKIM Ident List of mail server software List of SMTP server return codes POP before SMTP / SMTP after POP Internet Message Access Protocol Binary Content Extension Sender Policy Framework (SPF) Simple Authentication and Security Layer (SASL) SMTP Authentication Variable envelope return path Comparison of email clients for information about SMTP support Notes References External links SMTP Service Extensions Simple Mail Transfer Protocol SMTP Service Extension for Authentication (obsoletes ) SMTP and LMTP Transmission Types Registration (with ESMTPA) Message Submission for Mail (obsoletes , which obsoletes ) Internet mail protocols