I would be glad to help. You are correct in thinking that a bounce-back error mail would indicate the email address is invalid. So the email is being received. I would assume one of three scenarios is occuring.
1. The recipient has read your emails, but has decided not to respond to them.
2. The recipient no longer checks mail sent to the email account.
3. Your email address is being caught by the spam filter and they are not reaching their inbox.
There isn't much you can do if #1 or #2 is the case. However if it is #3 you can try using a different email address to reach this person. But if the receiver is intentionally ignoring you, I don't think a new email address would get them to respond. Since the email address is at yahoo, you could try creating a yahoo account and reaching them using yahoo messenger using the link below:
Please let me know once you've tried one of my suggestions above. There is no guarantee either of them will work as it depends on the other person choosing to respond to you. But let me know either way, and maybe we can think of another solution depending on what happens.
I would be glad to resolve this issue for you, but if you could let me know whether or not any of my previous suggestions were helpful, that would make is easier for me to provide a better solution. Please visit your question page here and let me know: