When you try to send a test message (assuming that the password is correct), you can get an error message "This User Does Not Have Access Rights To This Service".
This means that Yandex recognized you, but you do not have access to your mailbox through third-party services.
In order to solve it, you need to do the following:
Open the profile page in Yandex.Pasport. Scroll down to Passwords and authorization → Enable app passwords.
In the window that opens, select "Create a new password"
Select the type of application “Mail“
Enter an arbitrary name for a password and click the "Create" button.
In the window that opens, click the "Copy" button (1) and the "Done" button (2)
Insert the copied password in the ZennoPoster mail service properties
The next step is to go to the mail settings page. Mail → All settings → Email clients https://mail.yandex.com/#setup/client
Allow access to the mailbox from third-party mail clients and save the changes.
After that, repeat the test message sending. If everything is done correctly, then in the status bar you will see "Test email has been sent successfully".