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Hi. My name is***** you for your question and the opportunity to assist.
The problem is that Yahoo considers Quickbooks to be an unsafe program. To get Yahoo
to allow Quickbooks to send, from the Yahoo page click your name and open Account Info.
On the left select Account Security.
Enter your password, if requested and select Account Security again.
Scroll down and move the slider for "Allow apps that use less secure sign in"
such that it turns blue to turn it on.
Once it's on you should be able to send email via Quickbooks.