The only thing it can be is:
- The source and estinatimails are the same (e.g you send it to webmaster@yoursite.com while your site send emails from that address also.)
I created a second e-mail account for myself and it still doesn't send although it says it did.
taxick, I think we have the same issue, we solve one we solve both

. Keep you informed..
EDIT: Something must be wrong in my reputations. I changed the setting to -1 instead of 10 and now it works
"Minimal Reputation Required to Receive Changes in Number of Invites"