coopster - 10:56 pm on Mar 10, 2009 (gmt 0)

They may have a spam filter running on their inbound mail server that quietly discards your mail if it sees something it doesn't like. And if they are using an email address from their ISP, well, many ISPs today are running spam filters too. If your end user is merely checking their local client email spam filter folder rather than the server spam storage container (if it is indeed being stored, which many ISPs will do for a limited period of time), they won't notice your message. You have to track it from end-to-end, and yes, it can be a pain.