rhino_cole is referring to turning on the Yahoo! Mail security app feature, abottomlineco. 

I have here a detailed guide article that you can read through. These steps are applicable if you are using Gmail or Yahoo! Mail as your email provider: QuickBooks Will Not Accept My WebMail Password.

You'll also want to check this article if you have not set up email service. Once done, try to send an email again. 

We are always here to help you if you have other concerns.