Msg#: 3867463 posted 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.
Msg#: 3867463 posted 8:20 am on Mar 11, 2009 (gmt 0)
All those services have several layers of very aggressive filtering. Messages may not reach the layer that the recipient can control although I would have expected a bounce message in that case. Of course your own filters may be blocking the bounces.
As coopster says, although these are primarily webmail services they all have pop3 options these days but the spam folder is only accessible through webmail and is invisible to the client.