If you are using SSL try switching to None or vice versa. If your login is correct, try changing your outgoing encryption.
We have a guide for how to change your email password here. If you are unable to log in, a password reset for your email would be your best next step. You may need to adjust your Yahoo account to 'Allow apps that use less secure sign in' to get Outlook to connect: Ways to securely access Yahoo Mail. The best way to test this is to try and login via our webmail. In reply to RT Houston1's post on April 9, 2019. If you are still having trouble sending, make sure that the login details you’re using are correct. What if I am still having trouble sending? That’s it! You should now be able to send emails again. Once added, go to MacMail to login, make sure to use the temporary password provided from the yahoo account (steps below) and not your yahoo password. You will be prompted to add the new email manually. If such is the case then for a fix to this there are some solutions that you can try and see if that helps you with the fix of the issue. The easiest way to override this error is by deleting Yahoo from MacMail box and then add it under Other. Make sure any username and password fields are filled in if shown. Yahoo mail SMTP server not working- If in case you are not able to send or receive messages on your Yahoo mail service, then that can be because of the problem in the SMTP server settings workings.Check the current outgoing port and change it to 26 or 587.Locate the outgoing SMTP server and its settings.Open the settings for your email in the application or device you are using.Please Note: These steps can differ depending on the device and the application in use.