Try to avoid sending emails, instead publish those reports to folders or News Stand. If you runn reports succesfully that means that there is nothing wrong with NPrinting and we have to focus on your SMTP server
Once you confirm above - make sure you have smtp server configured properly. Talk to your mail admin and see if there is a proper setup done on smtp server settings in NPrinting Admin console, as well as whether SMTP server has any limits of emailes beeing sent per Hour / per Day. As NPrinting server may send a lot of them SMTP server can assume it is all junk mail and can block it from sending (although it is unlikely as it would not throw an error in NPrinintg logs, but only in SMTP queue)
do the first step and then we can take it frome there
BTW - i recommend upgrading to latest more stable NPrinitng version
cheers Lech, When applicable please mark the correct/appropriate replies as "solution" (you can mark up to 3 "solutions". Please LIKE threads if the provided solution is helpful to the problem.