We had a similar situation with a completely different program. The program in question sends out specialized messages to hundreds of clients based on selection criteria. Clients with a specific email provider (not Yahoo) were not receiving our emails. In our case, that email provider had blacklisted us thinking that we were spammers. We contacted the provider and explained what we are doing, and give them our domain info. they whitelisted us and we've been good ever since. Now in our case, like I say, we might be sending to 100+ people at the same time, I'm not sure that's your situation. But as mentioned by another, Yahoo , Hotmail and others have a trusted sender, if your email address is not in your customer's contact list, they may not come through
