To use the TagingAPI from Mozilla Firefox and Thunderbird, it will be the best, if the tags could be transmitted as comma separated list via httprequest.

Request from Mozilla Thunderbird:

• get me tags fitting to this email:
  • message-id
  • subject
  • body
• result:
  • list of:tag-uri + tag + class of tag

• get me tags of this email(s) (input either one e-mail or many) (has to be fast-as-hell)
  • message-id
• result:
  • list of:tag-uri + tag + class of tag

• tag this e-mail using that tag:
  • message-id
  • tag-uri
  • operation: add tag/remove tag
• result:
  • ok/error

• tag this e-mail using that tag-name:
  • message-id
  • tag-name (the label)
  • operation: add tag/remove tag
• result:
  • has to create a NEW TAG if tag-name does not exists, creates a URI for the tag.
  • return new tag-uri + ok/error

To identificate an email, we should use the Message-ID and not the mail-uri, because if an email is moved to another folder we will loss the relation of the tags to this mail.