I have recently been having troubling emailing statements only through Reckon Plus 2014

Hi,
I have recently been having troubling emailing statements only through Reckon Plus 2014. It takes a really long time or it won't send at all. I am emailing statements in a bulk amount (around 70) at a time. I tried emailing 3 or 4 at a time the next day and they send. However some statements when sent are coming back saying post master delivery failure - not sent to recipient; Yet I have emailed these customer before. This has been happening for the past month but I never had a problem before that. I recently upgraded the superannuation file so I was ready for superstream next year - I'm not sure if that has anything to do with it.