Turn on Less secure apps access

1. Login to your Yahoo Mail account via a web browser.
2. Go to the Account Info then to the Account Security page.
3. Turn the Allow apps that use less secure sign in setting to On

Turn on 2-Step Verification

1. Login to your Yahoo Mail account via a web browser.
2. Go to the Account Info then to the Account Security page.
3. Beside "Two-step verification," click the Toggle button to turn it on .
4. Enter your mobile number.
5. Click Send SMS or Call me to get a verification code.
6. Enter the verification code, then click Verify.

Create & use App Passwords

If 2-Step Authentication is enabled on your Yahoo account, you will need to create a third-party password just for Emailtree using these steps:

1. Login to your Yahoo Mail account via a web browser.
2. Go to the Account Info then to the Account Security page.
3. Scroll to the bottom and select Manage app passwords/Generate app password.
4. Choose Other app in the Select your app field and enter custom name: Emailtree, then select Generate.
5. Copy the password that is shown to a safe place. Select Done.

Set up Emailtree to access Gmail

You might need to provide the following details:

Your Account information

Full name or display name: Your name

User name: Your full email address

Password: Your Yahoo mail password

Incoming mail settings

Server: imap.mail.yahoo.com
Require SSL: Yes
Port: 993

Outgoing mail

Server: smtp.mail.yahoo.com
Require SSL: Yes
Require authentication: Yes
Port for SSL: 465 or 587

Source: https://help.yahoo.com/kb/add-two-step-verification-extra-security-sln5013.html