If you see the error message “535 5.7.8 Error: authentication failed:” in your logs then this means that somewhere in your email client’s setting there is an error.
Your mail server is expecting a specific username and password. While your are sending either an incorrect username or incorrect password (or both are incorrect). Re-check your login credentials for the email server that you are connecting to. If after troubleshooting and you can’t resolve this issue. You should reach out to your email host and request technical assistance. Some email web hosts will automatically block you if you have too many attempts with an incorrect password or username.