There may be work flow rules applied to the tracker that require specific fields to be set. As you can only define the artifact title and description via email, the artifact creation fails. If you wish to create artifacts in this tracker via email, the tracker admin will need to disable these work flow rules.
In addition, you may not be able to edit artifacts belonging to a tracker via email, if the tracker has two mandatory flex fields with parent-child relationship, but no default value relationship between the default values of the parent and child flex fields. In such cases, it is recommended to always establish the default value relationship between flex fields that have a parent-child relationship.