I recently used the (free) Google information bar to find out how recently Google had spidered a friend's website. Good news--they'd visited just a few days earlier, less than a week after the previous visit.
I cut and pasted the date and time of the visit into an email and sent it to my friend--and that's when the intrigue began. You see, my friend uses sbcglobal as her ISP, and SBC, as you may know, is Yahoo! (or owns Yahoo!, or is owned by Yahoo! or some such--I can't keep it all straight)
SBC sent me an autogenerated email telling me that my email was undeliverable for "policy reasons".
I puzzled over this for a moment, experienced a moment of gratitude for gmail (however creepy it might be that ads relating to the subject of my email pop up up in the margins) and considered sending my friend an invite to get a free gmail account. Then I re-sent the email to another of my friend's accounts.
She replied from her SBC account. That got through fine, trailing my cut and paste along with it, but when I tried to respond, I got another bounce message...policy reasons again. No explanation as to what policy I might have violated, of course. And I can't rule out the possibility that there's some code under that Google information that innocently set off some kind of screening. But I also can't rule out the possiblity that the policy I violated was one against consorting with the competition, and that idea leaves me decidedly uneasy.