In order to be able to send emails through your Yahoo account from Manager, you need to follow these steps to generate password for Manager.
Go to https://login.yahoo.com/account/security and click on
Generate app password
App passwords windows select
Other app, then enter
Manager app and click
Yahoo will generate new password which you can save somewhere.
Then within Manager, go to
Settings tab, then
Email Settings and enter following information:
Your Yahoo Address (e.g. email@example.com)
Your generated password for Manager app
Sending Limits: 500 Emails a day or Emails to 100 connections a day.