At the moment, when SMAX sends out notification to user, only one response is possible from user to one notification. If a user try to send a second notification , SMAX sends an error mail to user that the token has already been used.
This is very confusing to users because many times a user may want to send additional information via email to SMAX using the old email notification (that the user has already answered and therefore the token has been used and message never reaches SMAX).
I would suggest that the token provided to the user would be paired to the request itself, not to one specific notification sent to user. This way the user could use the same token over and over again and provide as much information as she/he wants to. When the request is closed, then the token would also expire.