IMAP Characteristics

This section is targeted at client developers or those with an interest in the lower-level IMAP details.

Conformance

Crymap fully conforms to the IMAP4rev1 (RFC 3501) and IMAP4rev2 (RFC 9051) standards. It also conforms to a number of extensions detailed in later sections.

Crymap passes the full Dovecot imaptest compliance suite excepting tests for extensions that Crymap does not implement.

Crymap was developed with the following priorities:

Don’t corrupt the user’s mail. (The “prime directive”.)
Be secure.
Conform to standards.
Be reasonably performant and light-weight.

In spite of the earlier paragraphs, there are cases where (1) and (3) come into conflict. Usually this is a result of a specification assuming all email messages are well-formed or pre-dating another specification change that makes compliance impossible without violating the prime directive, but there are also some issues with self-contradiction. These will all be discussed in individual sections where relevant.

Notwithstanding the few exceptions discussed, Crymap’s IMAP implementation conforms to the following standards:

RFC 2045 MIME Extensions: Format of Internet Message Bodies
RFC 2046 MIME Extensions: Media types
RFC 2047 MIME Extensions: Message Header Extensions for Non-ASCII Text
RFC 2157 UTF-7
RFC 2177 (IDLE)
RFC 2183 The Content-Disposition Header Field
RFC 2231 MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations
RFC 2342 (NAMESPACE)
RFC 2557 (Content-Location header field)
RFC 2595 (PLAIN authentication)
RFC 2971 (ID)
RFC 3066 Tags for the Identification of Languages
RFC 3282 Content Language Headers
RFC 3348 (CHILDREN)
RFC 3501 (IMAP4rev1)
RFC 3502 (MULTIAPPEND)
RFC 3516 (BINARY)
RFC 3691 (UNSELECT)
RFC 4315 (UIDPLUS)
RFC 4731 (ESEARCH)
RFC 4959 (SASL-IR)
RFC 4978 (COMPRESS=DEFLATE)
RFC 5161 (ENABLE)
RFC 5182 (SEARCHRES)
RFC 5253 (LIST-EXTENDED)
RFC 5322 (Internet Message Format)
RFC 5530 IMAP Response Codes
RFC 5819 (LIST-STATUS)
RFC 5918 Unicode Format for Network Interchange
RFC 6154 (CREATE-SPECIAL-USE and SPECIAL-USE)
RFC 6532 Internationalized Email Headers
RFC 6851 (MOVE)
RFC 6855 (UTF8=ACCEPT)
RFC 7162 (CONDSTORE and QRESYNC)
RFC 7888 (LITERAL+)
RFC 8438 (STATUS=SIZE)
RFC 8457 IMAP “$Important” Keyword and “\Important” Special-Use Attribute
RFC 8474 (OBJECTID)
RFC 9051 (IMAP4rev2)

Unicode support

Crymap is inherently a Unicode-aware, UTF-8-based application. All data which is returned to the client in a structured way (i.e., not as a raw byte stream) is internally managed as UTF-8.

If non-ASCII strings would be sent to a client that has not enabled UTF-8 support, one of two approaches is taken right before the data is put onto the wire, depending on the field:

The whole field may be put into RFC 2047 “encoded word” form, using base64-encoded UTF-8. Note that this means that returned “encoded words” are rarely in exactly the same form as they are in the original message, though the content remains the same.
Non-ASCII characters may be replaced by ‘X’. This is only done for fields that do not allow the previous strategy.

RFC 2184 is not implemented because it is impossible to implement its IMAP4 requirement at the same time as conforming to IMAP4rev1. Specifically, RFC 2184 would have the server decode encoded non-ASCII text in certain header parameters, but IMAP4rev1 does not allow these fields to contain non-ASCII content and the fields also may not contain encoded words. Instead, Crymap assumes the client will be able to deal with the parameters itself. This is likely to be a safe assumption, since this has always been an optional feature of IMAP servers, so clients need to be prepared to do it themselves either way.

Search uses Unicode “simple” case folding.

Limits

The maximum message size for APPEND is hard-wired to 64MB.

Batch insertions (through COPY, MOVE, or APPEND) are limited to 65536 messages.

Any operation that involves parsing message content will not process more than 20 levels of multipart or message/rfc822 nesting, nor will it process more than 1000 different body parts.

Message header lines in excess of 64kB are not processed.

Search operations will consider up to 128kB of text. Non-text body parts are ignored for search.

A mailbox can have up to 2’305’843’008 message IDs allocated. Note however that inserting multiple messages into a mailbox at once can use up to 128 times as many IDs as the number of messages actually inserted. Up to 3’999’999’999 change transactions can be performed on a mailbox. If either of these limits are reached, the mailbox will be “full” and further operations will fail. (Note that Crymap has not actually been tested in this condition.)

Mailboxes

All mailboxes other than INBOX may have children. Nesting depth is dependent on the host operating system’s maximum path length.

Mailbox names may not contain %, *, \, /, control characters or one of a few Unicode control characters, and may not begin with . or #.

Mailbox names other than INBOX are case-sensitive. INBOX is always upper-case.

The path delimiter is /. Making a mailbox path “absolute” by prefixing it with a / has no effect. Duplicate / characters in a name are ignored.

Non-normalised MUTF7 is accepted in mailbox names. Mailbox names are always returned in normalised MUTF7. MUTF7 is not returned on output nor transformed on input when the client has enabled UTF-8 support.

When UTF-8 support is enabled, including by way of IMAP4rev2, no normalisation of mailbox names occurs other than removal of extraneous path delimiters and the special case of INBOX being case-insensitive. IMAP4rev2 has a recommendation to send unsolicited LIST responses when a client uses a denormalised mailbox name. Due to the extremely limited utility of this feature in the context of Crymap’s implementation, Crymap does not implement this recommendation.

The \Marked and \Unmarked attributes are not used.

If a mailbox is deleted or renamed while a session has it open, the session will be terminated as soon as this occurrence is discovered.

When an account is created, the following mailboxes are made, with the shown special-use attributes.

Mailbox	Special-Use
`INBOX`
Archive	`\Archive`
Drafts	`\Drafts`
Sent	`\Sent`
Spam	`\Junk`
Trash	`\Trash`

Messages

Crymap tolerates and preserves messages with arbitrary binary content.

Clients in sessions that have not yet observed the expungement of a message can continue to access it for 24hr after the expungement.

If a message contains 8-bit content in the headers and the client requests the raw headers, Crymap will return that 8-bit content verbatim even if the client has not enabled UTF-8 support. If a message contains binary content and the client requests that binary content, even without using the BINARY extension, Crymap will return the binary content as-is because the IMAP specification gives no other way to return the data without corrupting it. Both of these issues are described in more detail in the descriptions of the UTF8=ACCEPT and BINARY extensions.

If messages contain non-DOS line endings, the line endings are returned as-is, so as not to corrupt anything. Crymap understands both UNIX and DOS line endings in messages. This is expected to be a non-issue, given that GMail actually served messages exclusively with UNIX line endings for years before anyone noticed. (Note though that when acting as a UNIX MDA, Crymap does convert line endings as they come in. This paragraph applies to messages that come in through IMAP or LMTP.)

Unsolicited FETCH responses always include UID and FLAGS. If CONDSTORE has been enabled, they include MODSEQ as well.

Flags

All “system flags” are supported and permanent. \Recent is fully implemented and fully atomic. Arbitrary keywords can be created without limit.

Authentication

LOGIN and AUTHENTICATE PLAIN are both supported and have the same effect. AUTHENTICATE does not allow the authorisation and authentication usernames to differ.

Extensions

RFC 8457

Crymap supports the $Important keyword and the \Important special-use attribute.

RFC 5530

Extended status codes are used everywhere they make sense.

APPENDLIMIT

Crymap uses a fixed APPENDLIMIT value which is reported in the capabilities list.

BINARY

Binary literals are understood, but are not handled any differently than non-binary literals. (That is, Crymap accepts and properly handles binary content in all contexts.)

If a client requests a BINARY body section, binary literal syntax is used iff the response payload contains a NUL byte.

The BINARY fetch feature is treated as orthogonal to the rest of the fetch options, and Crymap currently allows combinations not allowed by the standard.

The BINARY extension presents an odd conundrum: An IMAP4rev1 server is required to report the actual content transfer encoding of a binary part (binary), and is required to return body parts in their original transfer encoding when queried without BINARY, but at the same time is forbidden from returning binary data. In these cases, Crymap returns the binary data anyway, since doing so is a lesser evil than corrupting the data itself.

Crymap does not perform any encoding changes of messages appended using binary literals.

The standard insinuates that if the client requests content transfer decoding of a body part, and the server finds that the body part does not use DOS newlines, it should convert the newlines to DOS newlines. Crymap does not do this, since that constitutes data corruption.

CHILDREN

If a client makes a non-extended LIST command, \HasChildren and \HasNoChildren mailbox attributes are returned implicitly.

COMPRESS=DEFLATE

Deflate-based compression may be enabled at any time.

Crymap is not aware of whether TLS itself is using compression and so will not reject enabling compression even if TLS compression is active.

CONDSTORE

This extension is fully implemented.

Atomic updates are performed with respect to all flags on a message and not just the flags being modified, since this is both simpler and more useful. (And seems like what clients expect — there has been some talk of using conditional store to maintain tri-state flags like Junk/NonJunk/(nothing) properly.)

Modseqs are 63-bit integers as required by the later QRESYNC extension.

Internally, Modseqs are a 2-element vector clock pairing the maximum known UID with a monotonic “Change ID” (CID) with additional bookkeeping to maintain the extension’s strict ordering requirements. This can be seen in returned Modseq values as the UID multiplied by 4 billion plus the CID.

If multiple messages are changed at once, they all receive the same Modseq.

Expunging a message increments the highest Modseq value.

Crymap does not return HIGHESTMODSEQ response codes until CONDSTORE is enabled.

CREATE-SPECIAL-USE

The following special-use attributes are allowed: \Archive, \Drafts, \Flagged, \Junk, \Sent, \Trash, \Important. \All is not allowed since Crymap does not support an “all mail” view.

At most one special-use can be given to a mailbox. Crymap does not take action on special-use attributes except to return them, and does not prevent creating multiple mailboxes with the same special use.

ENABLE

This extension is fully implemented.

ESEARCH

This extension is fully implemented.

Crymap does not attempt to optimise searches for MIN or MAX alone.

ID

This extension is fully implemented.

The client-submitted identifying information is written into the server logs.

The [identification] section of the server configuration can provide additional attributes to be returned here. By default, Crymap returns name and version.

IDLE

This extension is fully implemented.

Notifications about changes are typically delivered within a millisecond of when they occurred (modulo client-server latency of course). Message creation and expungement and flag operations are all monitored.

Crymap does not strictly validate that the idle is terminated with “DONE”.

LIST-EXTENDED

This extension is fully implemented.

LIST-STATUS

This extension is fully implemented, but only because IMAP4rev2 requires it. No optimisations are made over simply making separate STATUS calls.

LITERAL+

This extension is fully implemented.

MOVE

This extension is fully implemented.

Moves occur as three separate atomic steps:

Messages are added to the destination.
Flags are set on the destination messages.
Messages are removed from the source.

MULTIAPPEND

This extension is fully implemented.

Setting flags on the messages occurs in a separate step message insertion. (This is allowed by the spec since even setting the flags at all is only a SHOULD.)

NAMESPACE

Crymap does not have namespaces. The extension is implemented in that it returns a canned “no namespaces” response.

OBJECTID

This extension is fully implemented except for the optional THREADID attribute, since Crymap does not support message threads.

Mailbox IDs always begin with M, except for INBOXs mailbox ID, which always begins with I. Email IDs always begin with E.

All mailboxes support these attributes.

QRESYNC

This extension is fully implemented.

Crymap remembers the most recent 1024 expunge events.

SASL-IR

This extension is fully implemented.

SEARCHRES

This extension is fully implemented.

SPECIAL-USE

This extension is fully implemented.

See CREATE-SPECIAL-USE for a reference on what attributes are supported.

STATUS=SIZE

This extension is fully implemented to the letter of the standard and not at all to the spirit. It is only implemented because IMAP4rev2 requires it.

Returning the size of a mailbox sounds like a useful operation. However, the standard does not permit returning the actual size. Instead, it requires the returned size to be greater than or equal to the sum of the sizes of the messages as they would be fetched in full.

Since determining the size of a message requires decrypting it, Crymap has no way to do this in a way that even resembles efficiency. Instead, it takes advantage of the fact that the returned size need only be an upper bound and determines the “size” by simply multiplying the message count by 4GB.

UIDPLUS

This extension is fully supported.

No mailboxes have the UIDNOTSTICKY attribute.

UNSELECT

This extension is fully implemented.

UTF8=ACCEPT

The useful part of this extension is implemented. Clients using the extension cannot observe aspects (arguably) unimplemented. To clients not using the extension, Crymap is the same as any IMAP implementation that does not support this extension.

Once a client enables this extension, mailbox names are returned and accepted in UTF-8, and MUTF7 interpretation no longer occurs. Envelope and body structure data is returned in UTF-8, with all encoded words decoded.

The “UTF-8 literals” added by this extension are handled no differently than normal literals. The standard suggests that the server should do something to “downgrade” UTF-8 messages for the sake of clients not using this extension. Crymap does not do this, since it would violate the prime directive, and is also not useful since nearly all clients are already prepared to encounter 8-bit characters where they are not formally allowed by the older 7-bit standard.

The standard requires that non-UTF8 literals containing 8-bit characters in the message headers are rejected. Crymap also does not do this, since it is actively harmful, leaving users without the ability to copy their existing messages into Crymap, and as mentioned in the previous paragraph, clients are able to deal with such messages anyway.

XCRY

Crymap-specific extension. This entails several commands:

XCRY PURGE

No arguments.

All expunged messages in the selected mailbox are removed from the host filesystem immediately, making them inaccessible to concurrent sessions that have not yet observed the expungement.

This is mainly used as part of Crymap’s internal test suite.

XCRY GET-USER-CONFIG

No arguments.

Retrieves the current user configuration.

Response:

* XCRY USER-CONFIG (capabilities...)
  internal-key-pattern
  external-key-pattern
  password-changed-datetime
  [astring astring...]

capabilities provides a list of valid tokens that can be passed to XCRY SET-USER-CONFIG.

XCRY SET-USER-CONFIG

Arguments: key value [key value [...]]

Updates the given key-value pairs in the configuration. Using this to “configure” PASSWORD is also how password changes are done.

Response:

* XCRY BACKUP-FILE "filename"

The returned filename should be shown to the user to let them know what file to use to undo the change.

XLIST

Implements the XLIST command, which was developed for GMail before LIST-EXTENDED was standardised. Some clients may still use this instead of the extended LIST command.

XVANQUISH

Crymap-specific extension.

Adds the XVANQUISH command. This command takes one argument, a UID sequence set. The given messages are immediately expunged without having to go through the \Deleted dance first.

This is not under the XCRY umbrella since it could be useful to others.

XYZZY

This is obviously a very important extension for GMail compatibility.

Miscellaneous

Crymap always returns CAPABILITY response codes to the OK used as a greeting upon connection and after successful authentication.

Crymap accepts UNIX line endings in IMAP, but always outputs DOS line endings.

Extensions not implemented

Extensions in this list were deliberately excluded because they were not useful or harmful.

ACL, LIST-MYRIGHTS, RIGHTS=

Since users are strictly bound to their own mailboxes, permissions don’t make much sense for Crymap.

CATENATE, URL-PARTIAL, URLAUTH, URLAUTH=BINARY

Not implemented due to higher complexity with low benefit.

URLAUTH is feasible to implement, as it would work by encoding the session key of the message in question in the auth string. This would probably be sufficient to make it work with BURL. However, it’s not a feature that the author would get use of any time soon.

CONVERT

Insanity.

It also looks like, quite possibly, literally nobody has ever implemented this for server or client.

I18NLEVEL=

Requires use of an out-dated, non-standard algorithm for Unicode collation and folding.

SORT, ESORT, THREAD

These concerns are much better handled by the client. Now that QRESYNC exists, clients can cheaply keep envelope data synchronised and not only do these operations themselves, but do them using up-to-date, standardised collation algorithms which take into account the user’s locale.

Secondarily, these add a decent amount of memory overhead and aren’t something the author would ever get use of.