[Feature]: tagging of messages

[datenheim]

Is your feature request related to a problem?

Allways was frustrated that mail clients never had an option for stable server side tagging of messages in multi-user scenarios.
Tagging eases searching and allow grouping that can not be created by search patterns.
It is also giving "visual anchors" when the search produces many results.

Describe the solution you'd like

1. Allow the users to add tags to messages.

2. Tags could be used to mark specific meaning, such as: customer, complaint, invoice, spam, ...

3. The list of available tags might be defined by the admin for global tags.
   3a. If users can define additional individual tags is an optional feature.
   3b. If tags are defined per mailbox or are really global needs discussion.

4. Global tags must be visible to all users, while users tags maybe are only visible to the individual.

5. If later the admin decides to add a tag globally that some users had on their private tag list before, no double tags should appear. Don't know how to handle this cleverly.

6. Tags should be available as search filter criterion as well.

7. Tags can be applied to a single message just viewed but also the all messages in a filtered list or search result.

8. Special tags like spam could allow admins to remove such messages from the archive to minimise storage consumtion and user annoyance. However the spam handling is worth a separate topic.

Additional context

Not mentioned are those topics:

• auto-tag messages that hit certain criteria,
  e.g. if a message is on this list of "from" addresses or domains (with wildcards) add tag "customer",
  or if subject or attachment file name contains "invoice,rechnung,mahnung,billing,accounting,balance" add tag "money"
• make tagging available via API, so that external tools can identify messages for more sophisticated tagging