I have had this problem before.. and usually it can take a while for a sponsor to return PM's. I've had probems with safemail and hushmail accounts not receiving bounceback emails, but for me, gmail works 75% of the time.
I'M NOT SAYING DO YOUR BUSINESS WITH GMAIL ... just saying, it seems to get bounceback emails MUCH quicker.. as long as you are putting 'list' in subject line.. just send 2 or 3 of same email from gmail.. you will get bounceback almost instantly.