SMTP stands for Simple Mail Transfer Protocol. SMTP is required to send emails from an email account. If your Yahoo SMTP server not working properly, follow the troubleshooting steps mentioned below.
On your computer, open the default browser and log on to the official Yahoo Sign In page.
Check if you can access your Yahoo account with the help of your login credentials.
If you have configured your Yahoo account on an email client such as Thunderbird, Outlook, or Apple Mail, check whether you have enabled POP access on your Yahoo webmail.
If you have not enabled it, log on to your Yahoo webmail and click the Settings icon at the top-right corner of the screen.
Select the Mail Options option from the drop-down menu.
Navigate to the left-side panel and click the POP & Forwarding tab.
Enable the 'Access Yahoo Mail via POP' radio button while fix the Yahoo SMTP server not working issue.
Next, select your profile name at the top-right corner of the web page and choose the Account Info option.
Click the Account Security tab from the left-side panel and toggle on the button next to the 'Allow apps that use less secure sign in' option.