I would ask the customer to give the time/date the mail was sent and look for into the Delivery logs in troubleshooting/logs section of SM admin to first see if SmarterMail really sent 14 times the same e-mail.
There should then be 14 outgoing SMTP sessions to the destination in the log.
If you find them, then it would be interresting to see if there is any error and if they all have the same message id or if it differs for each SMTP session.
This would be a good starting point to understand what is happening here.
Sébastien Riccio
System & Network Admin
https://swisscenter.com