email validation
Using Jamroom
You can use the EMAIL DELIVERY LOG on your server to see if the email got sent out or not. Yours is here:
https://www.jamroom.net/ibr/hosting/domain_email_log/338/id=1208?search_string=yahoo
We can see in that log that there have been emails delivered to yahoo. Could be that yahoo is putting the emails into the junk folder until some action is taken by their users, hard to guess at, could be anything.
If you are able to track down a repeatable structure to use for testing then it might be possible to understand more.
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