hi, have you checked with your hosting provider and asked them to check if their IP address is listed in the Anti-Spam database? so that the email won't arrived to Yahoo ? You should be able to send email to yahoo once the blacklisted IP is removed (if any).
I had this issue long time ago, my host provider's IP once got their IP blacklisted by SpamHaus (other users on same hosting server somehow abused the email services/spamming) and I couldn't send email to yahoo for few days - until the IP was removed upon request by my host provider.
It could be an IP address or domain name issue. Yahoo has been pretty aggressive in blocking a number of IP addresses. A lot of my clients, face the same issues (even emails from other email providers)
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