Virtual mailboxes

First, you'll have to load the plugin:

mail_plugins = $mail_plugins virtual

Namespace configuration

Then, you'll have to create a namespace for the virtual mailboxes, for example:

namespace {
  prefix = virtual/
  separator = /
  location = virtual:~/Maildir/virtual
}

After this you can create virtual mailboxes under ~/Maildir/virtual. By default it uses the "fs" layout, so you can create directories such as:

INBOX: ~/Maildir/virtual/INBOX/
Sub/mailbox: ~/Maildir/virtual/Sub/mailbox/

If you prefer to use Maildir++ layout instead, you can simply append :LAYOUT=maildir++ to the location.

Virtual mailbox configuration

For each virtual directory you need to create a dovecot-virtual file. Its syntax is like:

<1+ mailbox patterns>
  <search program>
[<more mailbox patterns>
  <search program for these mailboxes>
[etc..]]

Mailbox patterns can contain IMAP LIST-compatible "*" and "%" wildcards. They are currently evaluated only when the virtual mailbox is being selected, so if more mailboxes are created during that they aren't noticed.

"*" wildcard matches only one namespace at a time based on the namespace prefix. For example if you have namespaces with an empty prefix and a prefix "mail/":

* matches only mailboxes from the namespace with empty prefix
mail* matches mailboxes beginning with name "mail" from the namespace with empty prefix
mail/* matches only mailboxes from the mail/ namespace

Beware that "*" will not match any mailbox which already has a more specialised match!

The mailbox names have special prefixes:

'-': Don't include this mailbox.
'+': Drop \Recent flags from the backend mailbox when opening it.
'!': Save new mails to this mailbox (see below).

If you need to actually include a mailbox name that contains such prefix, you can currently just kludge it by using '+' prefix (if you don't care about the \Recent flags) and adding the mailbox name after that (e.g. "+-box").

Search program is compatible with IMAP SEARCH command. Besides the standard SEARCH key you may want to use X-MAILBOX key which matches the message's original mailbox. Note the leading whitespace in front of search specifications.

Saving mails to virtual mailboxes

It's possible to configure virtual mailbox so that it's possible to save/copy messages there. This is done by specifying a single physical mailbox where the message is really saved by prefixing it with '!', e.g.:

!INBOX
work/*
 unseen

Note however that nothing guarantees that the saved mail will actually show up in the virtual mailbox. If a message was saved with \Seen flag to the above virtual mailbox, it wouldn't show up there. This also means it's problematic to support IMAP UIDPLUS extension for virtual mailboxes, and currently Dovecot doesn't even try (no [APPENDUID] or [COPYUID] is sent to client).

The !-prefixed virtual mailbox is also selected from; you don't need to list it again without an ! or you'll get two copies of your messages in the virtual mailbox.

Sieve filters with virtual mailboxes

Using the sieve plugin with virtual mailboxes will cause dovecot to output a fatal exception error in it's logs and crash. This is because sieve can't tell the difference between a virtual location and a maildir/mbox location due to the way it detects actions in the mailboxes.

If you use virtual mailboxes that are configured in sieve, make sure that they point to the namespace which has a maildir/mbox location and a unique prefix. If you don't, sieve will crash trying to copy a message to a virtual mailbox.

Mailbox selection base on metadata (v2.2.22+)

Instead of a mailbox name, you can specify a metadata filter:

[-]/<metadata-entry-name>:<value-wildcard>

There can be multiple metadata entries. All the entries must match.

For example:

*
/private/vendor/vendor.dovecot/virtual:*
-/private/vendor/vendor.dovecot/virtual:ignore
  all

This matches all mailboxes, which contain a virtual METADATA entry that has any value except "ignore".

Examples

List all messages with \Deleted flag in all mailboxes:

# ~/Maildir/virtual/Trash/dovecot-virtual
*
  deleted

List all unseen INBOX and work/* messages:

# ~/Maildir/virtual/unseen/dovecot-virtual
INBOX
work/*
  unseen

Create a GMail-style "conversation view" for INBOX which shows all threads that have messages in INBOX, but shows all messages in the thread regardless of in what mailbox they physically exist in:

# ~/Maildir/virtual/all/dovecot-virtual
*
  all

# ~/Maildir/virtual/INBOX/dovecot-virtual
virtual/all
  inthread refs x-mailbox INBOX

Create a mailbox containing messages from all mailboxes except Trash and its children:

# ~/Maildir/virtual/all/dovecot-virtual
*
-Trash
-Trash/*
  all

Create a virtual Sentmail folder that includes Sent*:

# ~/Maildir/virtual/Sentmail/dovecot-virtual
Sent*
  all

List messages from past 48 hours (syntax is in seconds):

# ~/Maildir/virtual/recent/dovecot-virtual
INBOX
work/*
  all younger 172800

List unseen messages from foo and flagged messages from all mailboxes (including foo):

# ~/Maildir/virtual/example/dovecot-virtual
foo
  or unseen flagged
*
  flagged

Virtual POP3 INBOX

If you want POP3 INBOX to contain some or all mailboxes, you can do this in the following way:

Namespace configuration:

# The default namespace that is visible to IMAP clients
namespace inbox {
  prefix =
  separator = /
  list = yes
}

# Virtual namespace for the virtual INBOX. Use a global directory for dovecot-virtual files.
namespace virtual {
  prefix = virtual/
  separator = /
  location = virtual:/etc/dovecot/virtual:INDEX=~/Maildir/virtual
  list = no
  hidden = yes
}

# Copy of the inbox namespace. We'll use this in dovecot-virtual file.
namespace real {
  prefix = RealMails/
  separator = /
  list = no
  hidden = yes
}

Note that none of the namespaces have inbox=yes. This is because for IMAP users you want the inbox namespace to have inbox=yes, but for POP3 users you want the virtual namespace to have inbox=yes. This requires setting the inbox=yes in userdb extra fields. For example with MySQL you can can do this like:

user_query = SELECT ..., \
  CASE '%s' WHEN 'pop3' THEN NULL ELSE 'yes' END AS 'namespace/inbox/inbox', \
  CASE '%s' WHEN 'pop3' THEN 'yes' ELSE NULL END AS 'namespace/virtual/inbox' \
  WHERE ...

Finally specify what the virtual INBOX looks like for POP3 users:

/etc/dovecot/virtual/INBOX/dovecot-virtual :

RealMails
RealMails/*
-RealMails/Trash
-RealMails/Trash/*
-RealMails/Spam
  all

You'll have to use the RealMails/ prefix if you want to use "*" wildcard, otherwise it would match INBOX, which in turn would again lead to the virtual INBOX and that would create a loop.

Also to avoid accidental POP3 UIDL changes, you shouldn't base the UIDLs on IMAP UIDs. Instead use for GUIDs (with maildir the same as base filename):

pop3_uidl_format = %g