I know for sure that yahoo isn't blocking regular emails. I get mine without any problems from heliohost. You can try this simple test from your site and see what happens. Just make a php page out of the code below and add the correct email addresses and click onto the page.
<?php
$to = "your_address@yahoo.com";
$subject = "Test Mail";
$message = "Hello world!";
$headers = "From: raingirl@raingirl82.heliohost.org";
mail($to, $subject, $message, $headers);
echo "The email was sent";
?>