docs: Add example for customizing IMAP folders (#2045)

* docs: Add example for customizing IMAP folders (mailboxes)

* chore: Update `15-mailboxes.conf` to sync with upstream

This config has not been updated since 2016 (ignoring the Junk autosubscribe addition).

Synced to upstream equivalent at https://github.com/dovecot/core/blob/master/doc/example-config/conf.d/15-mailboxes.conf

Retains the `Archive` example definition from this PR and prior `auto = subscribe` additions.

---

Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>

README.md

@@ -176,6 +176,12 @@ If you got any problems with SPF and/or forwarding mails, give [SRS](https://git
 
 See the [documentation](https://docker-mailserver.github.io/docker-mailserver/edge/config/security/understanding-the-ports/) for further details and best practice advice, **especially regarding security concerns**.
 
+#### Mailboxes (_aka IMAP Folders_)
+
+`INBOX` is setup by default with the special IMAP folders `Drafts`, `Sent`, `Junk` and `Trash`. You can learn how to modify or add your own folders (_including additional special folders like `Archive`_) by visiting our docs page [_Customizing IMAP Folders_][docs-examples-imapfolders] for more information.
+
+[docs-examples-imapfolders]: https://docker-mailserver.github.io/docker-mailserver/edge/examples/use-cases/imap-folders
+
 ## Examples
 
 ### With Relevant Environmental Variables

docs/content/examples/uses-cases/imap-folders.md

@@ -0,0 +1,73 @@
+---
+title: 'Use Cases | Customize Mailbox Folders'
+hide:
+  - toc # Hide Table of Contents for this page
+---
+
+# Mailboxes (_aka IMAP Folders_)
+
+`INBOX` is setup as the private [`inbox` namespace][dovecot-docs-namespaces]. By default [`target/dovecot/15-mailboxes.conf`][gh-config-dovecot-mailboxes] configures the special IMAP folders `Drafts`, `Sent`, `Junk` and `Trash` to be automatically created and subscribed. They are all assigned to the private [`inbox` namespace][dovecot-docs-namespaces] (_which implicitly provides the `INBOX` folder_).
+
+These IMAP folders are considered special because they add a [_"SPECIAL-USE"_ attribute][rfc-6154], which is a standardized way to communicate to mail clients that the folder serves a purpose like storing spam/junk mail (`\Junk`) or deleted mail (`\Trash`). This differentiates them from regular mail folders that you may use for organizing.
+
+## Adding a mailbox folder
+
+See [`target/dovecot/15-mailboxes.conf`][gh-config-dovecot-mailboxes] for existing mailbox folders which you can modify or uncomment to enable some other common mailboxes. For more information try the [official Dovecot documentation][dovecot-docs-mailboxes].
+
+The `Archive` special IMAP folder may be useful to enable. To do so, make a copy of [`target/dovecot/15-mailboxes.conf`][gh-config-dovecot-mailboxes] and uncomment the `Archive` mailbox definition. Mail clients should understand that this folder is intended for archiving mail due to the [`\Archive` _"SPECIAL-USE"_ attribute][rfc-6154].
+
+With the provided [docker-compose.yml][gh-config-dockercompose] example, a volume bind mounts the host directory `config` to the container location `/tmp/docker-mailserver`. Config file overrides should instead be mounted to a different location as described in [Overriding Configuration for Dovecot][docs-config-overrides-dovecot]:
+
+```YAML
+volumes:
+  ...
+  - ./config/dovecot/15-mailboxes.conf:/etc/dovecot/conf.d/15-mailboxes.conf:ro
+```
+
+## Caution
+
+### Adding folders to an existing setup
+
+Handling of newly added mailbox folders can be inconsistent across mail clients:
+
+- Users may experience issues such as archived emails only being available locally.
+- Users may need to migrate emails manually between two folders.
+
+### Support for `SPECIAL-USE` attributes
+
+Not all mail clients support the `SPECIAL-USE` attribute for mailboxes (_defined in [RFC 6154][rfc-6154]_). These clients will treat the mailbox folder as any other, using the name assigned to it instead.
+
+Some clients may still know to treat these folders for their intended purpose if the mailbox name matches the common names that the `SPECIAL-USE` attributes represent (_eg `Sent` as the mailbox name for `\Sent`_).
+
+### Internationalization (i18n)
+
+Usually the mail client will know via context such as the `SPECIAL-USE` attribute or common English mailbox names, to provide a localized label for the users preferred language.
+
+Take care to test localized names work well as well.
+
+### Email Clients Support
+
+- If a new mail account is added without the `SPECIAL-USE` attribute enabled for archives:
+    - **Thunderbird** suggests and may create an `Archives` folder on the server.
+    - **Outlook for Android** archives to a local folder.
+    - **Spark for Android** archives to server folder named `Archive`.
+- If a new mail account is added after the `SPECIAL-USE` attribute is enabled for archives:
+    - **Thunderbird**, **Outlook for Android** and **Spark for Android** will use the mailbox folder name assigned.
+
+!!! caution "Windows Mail"
+
+    **Windows Mail** has been said to ignore `SPECIAL-USE` attribute and look only at the mailbox folder name assigned.
+
+!!! note "Needs citation"
+
+    This information is provided by the community.
+    
+    It presently lacks references to confirm the behaviour. If any information is incorrect please let us know! :smile:
+
+
+[docs-config-overrides-dovecot]: ../../config/advanced/override-defaults/dovecot.md#override-configuration
+[gh-config-dockercompose]: https://github.com/docker-mailserver/docker-mailserver/blob/master/docker-compose.yml
+[gh-config-dovecot-mailboxes]: https://github.com/docker-mailserver/docker-mailserver/blob/master/target/dovecot/15-mailboxes.conf
+[dovecot-docs-namespaces]: https://doc.dovecot.org/configuration_manual/namespace/#namespace-inbox
+[dovecot-docs-mailboxes]: https://doc.dovecot.org/configuration_manual/namespace/#mailbox-settings
+[rfc-6154]: https://datatracker.ietf.org/doc/html/rfc6154

docs/mkdocs.yml

@@ -147,6 +147,7 @@ nav:
       - 'Mailserver behind Proxy': examples/tutorials/mailserver-behind-proxy.md
     - 'Use Cases':
       - 'Forward-Only Mailserver with LDAP': examples/uses-cases/forward-only-mailserver-with-ldap-authentication.md
+      - 'Customize IMAP Folders': examples/uses-cases/imap-folders.md
   - 'FAQ' : faq.md
   - 'Contributing':
     - 'Issues and Pull Requests': contributing/issues-and-pull-requests.md

target/dovecot/15-mailboxes.conf

@@ -2,19 +2,50 @@
 ## Mailbox definitions
 ##
 
+# Each mailbox is specified in a separate mailbox section. The section name
+# specifies the mailbox name. If it has spaces, you can put the name
+# "in quotes". These sections can contain the following mailbox settings:
+#
+# auto:
+#   Indicates whether the mailbox with this name is automatically created
+#   implicitly when it is first accessed. The user can also be automatically
+#   subscribed to the mailbox after creation. The following values are
+#   defined for this setting:
+#
+#     no        - Never created automatically.
+#     create    - Automatically created, but no automatic subscription.
+#     subscribe - Automatically created and subscribed.
+#
+# special_use:
+#   A space-separated list of SPECIAL-USE flags (RFC 6154) to use for the
+#   mailbox. There are no validity checks, so you could specify anything
+#   you want in here, but it's not a good idea to use flags other than the
+#   standard ones specified in the RFC:
+#
+#     \All       - This (virtual) mailbox presents all messages in the
+#                  user's message store.
+#     \Archive   - This mailbox is used to archive messages.
+#     \Drafts    - This mailbox is used to hold draft messages.
+#     \Flagged   - This (virtual) mailbox presents all messages in the
+#                  user's message store marked with the IMAP \Flagged flag.
+#     \Important - This (virtual) mailbox presents all messages in the
+#                  user's message store deemed important to user.
+#     \Junk      - This mailbox is where messages deemed to be junk mail
+#                  are held.
+#     \Sent      - This mailbox is used to hold copies of messages that
+#                  have been sent.
+#     \Trash     - This mailbox is used to hold messages that have been
+#                  deleted.
+#
+# comment:
+#   Defines a default comment or note associated with the mailbox. This
+#   value is accessible through the IMAP METADATA mailbox entries
+#   "/shared/comment" and "/private/comment". Users with sufficient
+#   privileges can override the default value for entries with a custom
+#   value.
+
 # NOTE: Assumes "namespace inbox" has been defined in 10-mail.conf.
 namespace inbox {
-
-  #mailbox name {
-    # auto=create will automatically create this mailbox.
-    # auto=subscribe will both create and subscribe to the mailbox.
-    #auto = no
-
-    # Space separated list of IMAP SPECIAL-USE attributes as specified by
-    # RFC 6154: \All \Archive \Drafts \Flagged \Junk \Sent \Trash
-    #special_use =
-  #}
-
   # These mailboxes are widely used and could perhaps be created automatically:
   mailbox Drafts {
     auto = subscribe
@@ -35,17 +66,31 @@ namespace inbox {
     auto = subscribe
     special_use = \Sent
   }
-#  mailbox "Sent Messages" {
+
-#    special_use = \Sent
+  #mailbox "Sent Messages" {
-#  }
+  #  special_use = \Sent
+  #}
+
+  #mailbox Archive {
+  #  auto = subscribe
+  #  special_use = \Archive
+  #}
 
   # If you have a virtual "All messages" mailbox:
   #mailbox virtual/All {
   #  special_use = \All
+  #  comment = All my messages
   #}
 
   # If you have a virtual "Flagged" mailbox:
   #mailbox virtual/Flagged {
   #  special_use = \Flagged
+  #  comment = All my flagged messages
+  #}
+
+  # If you have a virtual "Important" mailbox:
+  #mailbox virtual/Important {
+  #  special_use = \Important
+  #  comment = All my important messages
   #}
 }