A list can be configured to have message topics (this notion is different from topics used to class mailing lists). Users can subscribe to these message topics in order to receive a subset of distributed messages: a message can have one or more topics and subscribers will receive only messages that have been tagged with a topic they are subscribed to. A message can be tagged automatically, by the message sender or by the list moderator.

Available message topics are defined by list parameters. For each new message topic, create a new msg_topic paragraph that defines the name and the title of the topic. If a thread is identified for the current message, then the automatic procedure is performed. Otherwise, to use automatic tagging, you should define keywords (see msg_topic). To define which part of the message is used for automatic tagging, you have to define the msg_topic_keywords_apply_on list parameter (see msg_topic_keywords_apply_on). Tagging a message can be optional or required, depending on the msg_topic_tagging list parameter.

This feature is only available with the normal delivery mode. Subscribers can select a message topic to receive messages tagged with this topic. To receive messages that were not tagged, users can subscribe to the topic other. The message topics selected by a subscriber are stored in the Sympa database (subscriber_table table).

First of all, if one or more msg_topic.keywords are defined, Sympa tries to tag messages automatically. To trigger manual tagging, by message sender or list moderator, on the web interface, Sympa uses authorization scenarios: if the resulting action is editorkey (for example in scenario send.editorkey), the list moderator is asked to tag the message. If the resulted action is request_auth (for example in scenario send.privatekey), the message sender is asked to tag the message. The following variables are available as scenario variables to customize tagging: topic, topic-sender, topic-editor, topic-auto, topic-needed (see Authorization scenarios). If message tagging is required and if it was not yet performed, Sympa will ask the list moderator.

Tagging a message will create a topic information file in the /home/sympa/spool/topic/ spool. Its name is based on the listname and the Message-ID. For message distribution, a X-Sympa-Topic field is added to the message, to allow members to use email filters.

If available, list members can select the TXT or HTML reception modes. In these modes, the list member will receive the selected version of a message if the message's content-type is multipart/alternative.