whilst many people appear to expect it, email offers no guarantee of delivery. No more than the postal service offer a guarantee. This has not been a major issue in the past as non delivery notification can back advising when this occured, which was one up on the postal service. However the sheer amounts of spam which purports to come from legitimate addresses has meant the helpless account holders received tens of thousands of these non delivery notices, one for every bad address in the spammers list. The rersult is almost all mail providers have turned off the NDR system and simply do not issue notices. To make matters worse major mail providers like Microsoft Yahoo and TWC (they are not alone) have implemented their own weird and wonderful spam rules. They not only refuse to send non delivery notices, but they refuse to deliver based on their own personal view of spam. A definition they refuse to share as spammer will use it to talor spam to get around their rules. Basically It may well have to do with the number of recipients. Do the originals get sent to many and the repeat to one email address perhaps?