RFCs Supported by Cyrus IMAP

The following is an inventory of RFCs supported by Cyrus IMAP.

RFC 822

Standard for the format of ARPA Internet text messages, obsoleted by RFC 2822.

RFC 0977

Network News Transfer Protocol

RFC 1036

Standard for interchange of USENET messages

RFC 1176

Interactive Mail Access Protocol: Version 2

RFC 1342

Representation of Non-ASCII Text in Internet Message Headers

RFC 1652

SMTP Service Extension for 8bit-MIMEtransport

RFC 1730

Internet Message Access Protocol - version 4, obsoleted by RFC 2060, RFC 2061, RFC 3501.

RFC 1869

SMTP Service Extensions

RFC 1870

SMTP Service Extension for Message Size Declaration

RFC 1939

Post Office Protocol - Version 3 (POP3)

RFC 1951

DEFLATE Compressed Data Format Specification version 1.3

RFC 1952

GZIP file format specification version 4.3

RFC 2033

Local Mail Transfer Protocol

RFC 2034

SMTP Service Extension for Returning Enhanced Error Codes

RFC 2045

Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies

RFC 2046

Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types

RFC 2047

MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text

RFC 2060

Internet Message Access Protocol - Version 4rev1, obsoleted by RFC 3501.

RFC 2086

IMAP4 ACL Extension, obsoleted by RFC 4314.


Backwards compatibility with this RFC is to be obsoleted.

RFC 2087

IMAP4 QUOTA extension

RFC 2088

IMAP4 non-synchronizing literals

RFC 2177

IMAP4 IDLE command

RFC 2192

IMAP URL Scheme, obsoleted by RFC 5092.

RFC 2193

IMAP4 Mailbox Referrals

RFC 2195

IMAP/POP AUTHorize Extension for Simple Challenge/Response

RFC 2246

The TLS Protocol Version 1.0

RFC 2298

Extensible Message Format for Message Disposition Notifications (MDNs)

RFC 2342

IMAP4 Namespace

RFC 2359

IMAP4 UIDPLUS extension, obsoleted by RFC 4315

RFC 2425

A MIME Content-Type for Directory Information

RFC 2426

vCard MIME Directory Profile

RFC 2444

The One-Time-Password SASL Mechanism

RFC 2449

POP3 Extension Mechanism

RFC 2518

HTTP Extensions for Distributed Authoring -- WEBDAV

RFC 2595

Using TLS with IMAP, POP3 and ACAP

RFC 2617

HTTP Authentication: Basic and Digest Access Authentication, updated by RFC 7615, RFC 7616, RFC 7617.

RFC 2817

HTTP Upgrading to TLS Within HTTP/1.1

RFC 2818


RFC 2821

Simple Mail Transfer Protocol

RFC 2822

Internet Message Format

RFC 2831

Using Digest Authentication as a SASL Mechanism

RFC 2920

SMTP Service Extension for Command Pipelining

RFC 2971

IMAP4 ID extension

RFC 2980

Common NNTP Extensions

RFC 3028

Sieve: A Mail Filtering Language

RFC 3206

The SYS and AUTH POP Response Codes

RFC 3207

SMTP Service Extension for Secure SMTP over TLS

RFC 3253

Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning)

RFC 3339

Date and Time on the Internet: Timestamps

RFC 3348

IMAP4 Child Mailbox Extension

RFC 3431

Sieve Extension: Relational Tests

RFC 3463

Enhanced Mail System Status Codes

RFC 3501

Internet Message Access Protocol - version 4rev1

RFC 3502


RFC 3516

IMAP4 Binary Content Extension

RFC 3598

Sieve Email Filtering -- Subaddress Extension, obsoleted by RFC 5233.

RFC 3656

MUPDATE Protocol (For Cyrus Murder)

RFC 3691

Internet Message Access Protocol (IMAP) UNSELECT command

RFC 3744

Web Distributed Authoring and Versioning (WebDAV) Access Control Protocol

RFC 3834

Recommendations for Automatic Responses to Electronic Mail

RFC 3848

ESMTP and LMTP Transmission Types Registration

RFC 3894

Sieve Extension: Copying Without Side Effects

RFC 3977

Network News Transfer Protocol (NNTP)

RFC 4287

The Atom Syndication Format

RFC 4314

IMAP4 Access Control List (ACL) Extension

RFC 4315

Internet Message Access Protocol (IMAP) - UIDPLUS extension

RFC 4331

Quota and Size Properties for Distributed Authoring and Versioning (DAV) Collections

RFC 4346

The Transport Layer Security (TLS) Protocol Version 1.1

RFC 4422

Simple Authentication and Security Layer (SASL)

RFC 4466

Collected Extensions to IMAP4 ABNF

RFC 4467

Internet Message Access Protocol (IMAP) - URLAUTH Extension, updated by RFC 5092.

RFC 4469

Internet Message Access Protocol (IMAP) CATENATE Extension

RFC 4505

Anonymous Simple Authentication and Security Layer (SASL) Mechanism

RFC 4550

Internet Email to Support Diverse Service Environments (Lemonade) Profile

RFC 4551

IMAP Extension for Conditional STORE Operation or Quick Flag Changes Resynchronization

RFC 4559

SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows

RFC 4616

The PLAIN Simple Authentication and Security Layer (SASL) Mechanism

RFC 4642

Using Transport Layer Security (TLS) with Network News Transfer Protocol (NNTP)

RFC 4643

Network News Transfer Protocol (NNTP) Extension for Authentication

RFC 4644

Network News Transfer Protocol (NNTP) Extension for Streaming Feeds

RFC 4731

IMAP4 Extension to SEARCH Command for Controlling What Kind of Information Is Returned

RFC 4791

Calendaring Extensions to WebDAV (CalDAV)

RFC 4918

HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)

RFC 4954

SMTP Service Extension for Authentication

RFC 4959

IMAP Extension for Simple Authentication and Security Layer (SASL) Initial Client Response

RFC 4978


RFC 5032

WITHIN Search Extension to the IMAP Protocol

RFC 5034

The Post Office Protocol (POP3) Simple Authentication and Security Layer (SASL) Authentication Mechanism

RFC 5092

IMAP URL Scheme, updated by RFC 5593.

RFC 5161

The IMAP ENABLE Extension

RFC 5162

IMAP4 Extensions for Quick Mailbox Resynchronization

RFC 5173

Sieve Email Filtering: Body Extension

RFC 5182

IMAP Extension for Referencing the Last SEARCH Result

RFC 5183

Sieve Email Filtering: Environment Extension

RFC 5228

Sieve: A Mail Filtering Language

RFC 5229

Sieve Email Filtering: Variables Extension

RFC 5230

Sieve Email Filtering: Vacation Extension

RFC 5231

Sieve Email Filtering: Relational Extension

RFC 5232

Sieve Email Filtering: Imap4flags Extension

New in version 2.5.0.

RFC 5233

Sieve Email Filtering: Subaddress Extension

RFC 5235

Sieve Email Filtering: Spamtest and Virustest Extensions

RFC 5256

Internet Message Access Protocol - SORT and THREAD Extensions

RFC 5257

Internet Message Access Protocol - ANNOTATE Extension

RFC 5258

Internet Message Access Protocol version 4 - LIST Command Extensions

RFC 5260

Sieve Email Filtering: Date and Index Extensions

New in version 2.5.0.

RFC 5267

Contexts for IMAP4


The ESORT capability is implemented. The CONTEXT=SEARCH and CONTEXT=SORT capabilities are not implemented.

RFC 5293

Sieve Email Filtering: Editheader Extension

RFC 5321

Simple Mail Transfer Protocol

RFC 5322

Internet Message Format


The JMAP mapping is incomplete.

RFC 5397

WebDAV Current Principal Extension

RFC 5423

Internet Message Store Events

RFC 5429

Sieve Email Filtering: Reject and Extended Reject Extensions

RFC 5435

Sieve Email Filtering: Extension for Notifications

RFC 5436

Sieve Notification Mechanism: mailto

RFC 5463

Sieve Email Filtering: Ihave Extension

RFC 5464


RFC 5490

The Sieve Mail-Filtering Language -- Extensions for Checking Mailbox Status and Accessing Mailbox Metadata

RFC 5524

Extended URLFETCH for Binary and Converted Parts

RFC 5536

Netnews Article Format

RFC 5537

Netnews Architecture and Protocols

RFC 5545

Internet Calendaring and Scheduling Core Object Specification (iCalendar)

RFC 5546

iCalendar Transport-Independent Interoperability Protocol (iTIP)

RFC 5550

The Internet Email to Support Diverse Service Environments (Lemonade) Profile


The URL-PARTIAL capability is implemented. The CONTEXT=SEARCH, CONTEXT=SORT, CONVERT, I18NLEVEL=1, and NOTIFY capabilities are not implemented.

RFC 5593

Internet Message Access Protocol (IMAP) - URL Access Identifier Extension

RFC 5689

Extended MKCOL for Web Distributed Authoring and Versioning (WebDAV)

RFC 5703

Sieve Email Filtering: MIME Part Tests, Iteration, Extraction, Replacement, and Enclosure

RFC 5804

A protocol for Remotely Managing Sieve Scripts

RFC 5819

IMAP4 Extension for Returning STATUS Information in Extended LIST

RFC 5957

Display-Based Address Sorting for the IMAP4 SORT Extension

RFC 5995

Using POST to Add Members to Web Distributed Authoring and Versioning (WebDAV) Collections

RFC 6009

Sieve Email Filtering: Delivery Status Notifications and Deliver-By Extensions

RFC 6047

iCalendar Message-Based Interoperability Protocol (iMIP)

RFC 6101

The Secure Sockets Layer (SSL) Protocol Version 3.0


SSLv3 is considered insecure as it is vulnerable to POODLE.

Support for SSLv3 is being deprecated and removed.

RFC 6131

Sieve Vacation Extension: "Seconds" Parameter

RFC 6134

Sieve Extension: Externally Stored Lists

RFC 6154

IMAP LIST Extension for Special-Use Mailboxes


The LIST and LSUB commands return the special-use flags, unless the specialusealways configuration variable is explicitly turned off.

RFC 6203

IMAP4 Extension for Fuzzy Search

RFC 6321

xCal: The XML Format for iCalendar

RFC 6350

vCard Format Specification

RFC 6352

CardDAV: vCard Extensions to Web Distributed Authoring and Versioning (WebDAV)

RFC 6376

DomainKeys Identified Mail (DKIM) Signatures

RFC 6558

Sieve Extension for Converting Messages before Delivery

RFC 6455

The WebSocket Protocol

RFC 6578

Collection Synchronization for Web Distributed Authoring and Versioning (WebDAV)

RFC 6585

Additional HTTP Status Codes

RFC 6609

Sieve Email Filtering: Include Extension

RFC 6638

Scheduling Extensions to CalDAV

RFC 6764

Locating Services for Calendaring Extensions to WebDAV (CalDAV) and vCard Extensions to WebDAV (CardDAV)

RFC 6797

HTTP Strict Transport Security (HSTS)

RFC 6851

Internet Message Access Protocol (IMAP) - MOVE Extension

New in version 2.5.0.

RFC 6901

JavaScript Object Notation (JSON) Pointer

RFC 7230

Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing

RFC 7231

Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content

RFC 7232

Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests

RFC 7233

Hypertext Transfer Protocol (HTTP/1.1): Range Requests

RFC 7234

Hypertext Transfer Protocol (HTTP/1.1): Caching

RFC 7235

Hypertext Transfer Protocol (HTTP/1.1): Authentication

RFC 7238

The Hypertext Transfer Protocol Status Code 308 (Permanent Redirect)

RFC 7239

Forwarded HTTP Extension

RFC 7240

Prefer Header for HTTP

RFC 7265

jCal: The JSON Format for iCalendar

RFC 7352

Sieve Email Filtering: Detecting Duplicate Deliveries

RFC 7377

IMAP4 Multimailbox SEARCH Extension

RFC 7529

Non-Gregorian Recurrence Rules in the Internet Calendaring and Scheduling Core Object Specification (iCalendar)

RFC 7540

Hypertext Transfer Protocol Version 2 (HTTP/2)

RFC 7615

HTTP Authentication-Info and Proxy-Authentication-Info Response Header Fields

RFC 7616

HTTP Digest Access Authentication

RFC 7617

The 'Basic' HTTP Authentication Scheme

RFC 7692

Compression Extensions for WebSocket

RFC 7694

Hypertext Transfer Protocol (HTTP) Client-Initiated Content-Encoding

RFC 7725

An HTTP Status Code to Report Legal Obstacles

RFC 7804

Salted Challenge Response HTTP Authentication Mechanism

RFC 7808

Time Zone Data Distribution Service

RFC 7809

CalDAV: Time Zones by Reference

RFC 7889


RFC 7932

Brotli Compressed Data Format

RFC 7953

Calendar Availability

RFC 7986

New Properties for iCalendar


Support here means, that when the iCalendar stream is retrieved with HTTP GET, Cyrus IMAP inserts the color, description and name from the WebDAV properties. IMAGE, SOURCE, multi-lingual calendar DESCRIPTIONs, URL, LAST-MODIFIED, CATEGORIES, and REFRESH-INTERVAL are not exported on iCalendar streams retrieved with GET.

Individual iCalendar objects (VEVENT, VTODO, VJOURNAL) can be uploaded and downloaded with the New Properties for iCalendar.

RFC 8144

Use of the Prefer Header Field in Web Distributed Authoring and Versioning (WebDAV)

RFC 8246

HTTP Immutable Responses

RFC 8288

Web Linking

RFC 8297

An HTTP Status Code for Indicating Hints

RFC 8437

IMAP UNAUTHENTICATE Extension for Connection Reuse

RFC 8438

IMAP Extension for STATUS=SIZE

RFC 8440

IMAP4 Extension for Returning MYRIGHTS Information in Extended LIST

RFC 8441

Bootstrapping WebSockets with HTTP/2

RFC 8474

IMAP Extension for Object Identifiers

RFC 8514

Internet Message Access Protocol (IMAP) - SAVEDATE Extension

RFC 8579

Sieve Email Filtering: Delivering to Special-Use Mailboxes

RFC 8580

Sieve Extension: File Carbon Copy (FCC)

RFC 8607

Calendaring Extensions to WebDAV (CalDAV): Managed Attachments

RFC 8620

The JSON Meta Application Protocol (JMAP)

RFC 8621

The JSON Meta Application Protocol (JMAP) for Mail

RFC 8878

Zstandard Compression and the application/zstd Media Type

RFC 8887

A JSON Meta Application Protocol (JMAP) Subprotocol for WebSocket

RFC 8970

IMAP4 Extension: Message Preview Generation



Sieve Email Filtering: delivery by mailboxid


Sieve Email Filtering: Snooze Extension


JMAP for Calendars


JMAP for Sieve Scripts


LMTP Service Extension for Ignoring Recipient Quotas

[MS-NTHT] NTLM Over HTTP Protocol Specification


Sieve Email Filtering -- Regular Expression Extension


Sieve -- An extension for providing instant notifications


VPOLL: Consensus Scheduling Component for iCalendar


Internet Calendar Scheduling Protocol (iSchedule)


Hypertext Transfer Protocol (HTTP) Keep-Alive Header

caldav-ctag Calendar Collection Entity Tag (CTag) in CalDAV Brief Header Microsoft 'Brief' header extension

RFC Wishlist

RFC 2221

IMAP4 Login Referrals

RFC 2295

Transparent Content Negotiation in HTTP

RFC 2369

The Use of URLs as Meta-Syntax for Core Mail List Commands and their Transport through Message Header Fields

RFC 3229

Delta encoding in HTTP

RFC 5255

Internet Message Access Protocol Internationalization

RFC 5259

Internet Message Access Protocol - CONVERT Extension

RFC 5437

Sieve Notification Mechanism: Extensible Messaging and Presence Protocol (XMPP)

RFC 5465

The IMAP NOTIFY Extension

RFC 5466

IMAP4 Extension for Named Searches (Filters)

RFC 5842

Binding Extensions to Web Distributed Authoring and Versioning (WebDAV)

RFC 6468

Sieve Notification Mechanism: SIP MESSAGE

RFC 6785

Support for Internet Message Access Protocol (IMAP) Events in Sieve

RFC 6855

IMAP Support for UTF-8

RFC 7162

IMAP Extensions: Quick Flag Changes Resynchronization (CONDSTORE) and Quick Mailbox Resynchronization (QRESYNC)

RFC 7888

IMAP4 Non-synchronizing Literals

RFC 8470

Using Early Data in HTTP

RFC 8508