Ok,
So if it's the same people all the time, then we need to do some basic troubleshooting:
Is there a comanality between their email servers? Ie are they all Gmail, are they all hosted by the same email provider?
If so, are there others on the list that use the same provider?
Also if the email is sent to just one of the problematic emails does it go through? If so the anti spam may not like the amount of receipents.
Clint