Here's what I've observed. It doesn't directly apply to your problem, but may shed a little light: My company sends out a lot of emails to our users. Yahoo wil

Here's what I've observed. It doesn't directly apply to your problem, but may shed a little light: My company sends out a lot of emails to our users. Yahoo will occasionally, for no known reason, put our first email into spam. From then on, unless the user explicitly marks that email as not spam, yahoo bounces all future emails sent to that user with a "mailbox doesn't exist" message. I've repeatedly had to do phone support where I directed the user to their spam folder to find our very first email. Once that gets ok'd we no longer have any trouble with that specific user. On the topic of hotmail, this article is interesting: http://hubpages.com/hub/Hotmail_Fails_To_Deliver_Up_To_81_Of_All_Attachment_Emails http://hubpages.com/hub/Hotmail_Fails_To_Deliver_Up_To_81_Of...