Yes, you are absolutly right. I have done a new set of tests using new buyers email addresses and the thank-you emails are there as they should. I will have to carry on trying to debug the problem meself, but something is sure, the problem is not at your end.
I suspect the problem is at Yahoo email service. I use Yahoo email Plus, and most of my website email addresses are forwarded there. I think some of the emails are lost without any apparent reason. By the way, is not the first time I have issues with this company. I guess it is time to move to a more reliable email service provider.
Thank you so much for excelent support and patience.