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.