Mail

Mail is first allocated with mail_alloc(). Mails always belong to a transaction. Even if mail is treated read-only, Dovecot might write data to cache file, so whenever possible, mail transactions should be committed. When mail is allocated, you can specify a list of fields and headers that you're (most likely) going to need. This allows Dovecot to optimize the later mail_get_*() lookups so that it doesn't need to parse the message multiple times. These fields are also added to cache file, so you shouldn't list fields unless you're fairly certain you need them.

Usually the mails are created and returned by mailbox_search_*() functions. This is preferable even if you're doing a simple search, such as "all mails". Using the search API allows the internal implementation to prefetch mails, which can reduce latency. Alternatively you can use mail_set_seq() and mail_set_uid() to jump between mails, but this prevents prefetching.

Getting Data

Mail Fields

Some of the mail fields can be accessed directly:

box

Mail's mailbox, same as the transaction's.

transaction

Mail's transaction, the same that was given to mail_alloc().

seq

Currently selected message's sequence number.

uid

Currently selected message's UID.

expunged

We already detected that the message is expunged and can't be accessed. This may also be set (and looked up) later when some mail_get_*() function fails.

has_nuls

has_no_nuls

Message body is known to (not) have NUL characters.

lookup_abort

The final field is lookup_abort, which is write-only. Normally when doing a mail_get_*() operation for a field that isn't in a cache, the field is generated and added to cache. If you don't want this, but instead have figured out some better optimized way to do non-cached lookups, you can change this field so that mail_get_*() lookups fail instead with MAIL_ERROR_LOOKUP_ABORTED. This is primarily used by searching code internally.

Lookup Abort Flags

MAIL_LOOKUP_ABORT_NEVER

The default - do whatever it takes to return the value.

MAIL_LOOKUP_ABORT_READ_MAIL

If returning the value would require reading message headers/body, abort.

MAIL_LOOKUP_ABORT_NOT_IN_CACHE

If the value isn't already in cache, abort.

For example if looking up message's physical size would require stat()\ ing the file this wouldn't be done.

Mail Get Functions

Most of the mail_get_*() should be fairly obvious in how they work. Only functions returning int can fail, others don't.

• Keywords can be looked up either by getting an array of keyword strings or keyword indexes. The index lookups are slightly faster. Keyword indexes can be converted to strings by using the mailbox_status.keywords array returned by mailbox_get_status().

• mail_get_first_header() returns 0 if header wasn't found, 1 if it was.

• mail_get_special() can return various special fields. If a special isn't implemented by some backend, the call returns success and sets the value to empty string.

• mail_get_stream() returns an input stream that can be used to access the mail. If this function is called multiple times, each call seeks the stream to beginning. Don't unreference the stream, it's destroyed automatically.

  • Typically the input contains the raw data in disk, lines may end with LF or CRLF depending on how they're on disk.

  • mbox drops Dovecot's internal headers from the stream (X-UID:, Status:, etc.).

  • Plugins (e.g. mail-compress plugin) can also hook into this call and modify the input stream.

Sometimes you might notice that some looked up field is actually corrupted. For example you might notice that input stream returns EOF earlier than previous mail_get_physical_size() said. This might have been caused by various different things, but in any case all you can really do then is to just call mail_set_cache_corrupted() and try again.

Changing Metadata

Some of the messages' metadata can be updated:

• mail_update_flags() and mail_update_keywords() changes flags/keywords.

  • Usually you should set modify_type parameter to MODIFY_ADD or MODIFY_REMOVE, instead of replacing all the flags. That allows concurrent flag updates not to overwrite each others changes.

• mail_expunge()` expunges a message.

Other functions are mainly intended for mailbox replication or restoring an existing mailbox (e.g. dsync):

• mail_update_modseq() and mail_update_pvt_modseq() can be used to increase the message's shared/private modseq.

• mail_update_pop3_uidl() can be used to give a specific POP3 UIDL for the message. This is used internally when pop3_save_uidl=yes.

Other metadata can't be changed. IMAP protocol requires that messages are immutable, so it's not possible to change a message's received date, headers or body. If you wish to modify any of them, you need to create a new message and expunge the old one.