I'm no expert, but most ISPs have a way to check for messages online (as
opposed to downloading them onto your computer). (For example, with a
Verizon email address, I can go to netmail.verizon.net and check my emails
there.
Your recipient could check to see if you messages are there on his ISPs
server. If they are, that would suggest that they're getting intercepted in
the process of being downloaded from the ISP server to his computer...which
would then suggest there's some spam or antivirus program on his computer
that's interfering.
On the other hand, if the messages aren't even showing up on the ISPs
server, then it would seem that the ISP is intercepting them...
Hope this helps.
Ken Isaacson
SILENT COUNSEL, a legal thriller
www.KenIsaacson.com