| 1845 | SMTP Service Extension for Checkpoint/RestartD. Crocker, N. Freed, A. Cargille | Experimental | September 1995 |
| 1846 | SMTP 521 Reply CodeA. Durand, F. Dupont | Experimental | September 1995 |
| 1870 | SMTP Service Extension for Message Size DeclarationJ. Klensin, N. Freed, K. Moore | Internet Standard | November 1995 |
| 1939 | Post Office Protocol - Version 3J. Myers, M. Rose | Internet Standard | May 1996 |
| 1985 | SMTP Service Extension for Remote Message Queue StartingJ. De Winter | Proposed Standard | August 1996 |
| 2034 | SMTP Service Extension for Returning Enhanced Error CodesN. Freed | Proposed Standard | October 1996 |
| 2119 | Key words for use in RFCs to Indicate Requirement LevelsS. Bradner | Best Current Practice | March 1997 |
| 2476 | Message SubmissionR. Gellens, J. Klensin · obsoleted by RFC 4409 | Proposed Standard | December 1998 |
| 2645 | ON-DEMAND MAIL RELAY (ODMR) SMTP with Dynamic IP AddressesR. Gellens | Proposed Standard | August 1999 |
| 2852 | Deliver By SMTP Service ExtensionD. Newman | Proposed Standard | June 2000 |
| 2920 | SMTP Service Extension for Command PipeliningN. Freed | Internet Standard | September 2000 |
| 3030 | SMTP Service Extensions for Transmission of Large and Binary MIME MessagesG. Vaudreuil | Proposed Standard | December 2000 |
| 3207 | SMTP Service Extension for Secure SMTP over Transport Layer SecurityP. Hoffman | Proposed Standard | February 2002 |
| 3461 | Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs)K. Moore | Draft Standard | January 2003 |
| 3463 | Enhanced Mail System Status CodesG. Vaudreuil | Draft Standard | January 2003 |
| 3501 | INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1M. Crispin · obsoleted by RFC 9051 | Proposed Standard | March 2003 |
| 3865 | A No Soliciting Simple Mail Transfer Protocol (SMTP) Service ExtensionC. Malamud | Proposed Standard | September 2004 |
| 3885 | SMTP Service Extension for Message TrackingE. Allman, T. Hansen | Proposed Standard | September 2004 |
| 4141 | SMTP and MIME Extensions for Content ConversionK. Toyoda, D. Crocker | Proposed Standard | November 2005 |
| 4301 | Security Architecture for the Internet ProtocolS. Kent, K. Seo | Proposed Standard | December 2005 |
| 4409 | Message Submission for MailR. Gellens, J. Klensin · obsoleted by RFC 6409 | Draft Standard | April 2006 |
| 4880 | OpenPGP Message FormatJ. Callas, L. Donnerhacke, H. Finney, D. Shaw, R. Thayer · obsoleted by RFC 9580 | Proposed Standard | November 2007 |
| 4954 | SMTP Service Extension for AuthenticationR. Siemborski, Ed., A. Melnikov, Ed. | Proposed Standard | July 2007 |
| 5321 | Simple Mail Transfer ProtocolJ. Klensin | Draft Standard | October 2008 |
| 5322 | Internet Message FormatP. Resnick, Ed. | Draft Standard | October 2008 |
| 5741 | RFC Streams, Headers, and BoilerplatesL. Daigle, Ed., O. Kolkman, Ed., IAB · obsoleted by RFC 7841 | Informational | December 2009 |
| 5751 | Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message SpecificationB. Ramsdell, S. Turner · obsoleted by RFC 8551 | Proposed Standard | January 2010 |
| 6152 | SMTP Service Extension for 8-bit MIME TransportJ. Klensin, N. Freed, M. Rose, D. Crocker, Ed. | Internet Standard | March 2011 |
| 6376 | DomainKeys Identified Mail (DKIM) SignaturesD. Crocker, Ed., T. Hansen, Ed., M. Kucherawy, Ed. | Internet Standard | September 2011 |