Its just a matter of entering your user name and password. In some cases its something as simple as just checking that checkbox saying to use the same username and password for outgoing mail if you are using outlook or something like that. Check if your outlook has username and password authentication for outgoing messages.
If you received this message in relation to a script not working, then my guess is that you have provided incorrect or unneeded authentication (relay) to your mail server, which is refusing to accept your mail. You will need to ask your mail server host to check their logs, or provide the error code you are receiving, as we don't yet have enough information on the issue to propose a solution.
Please check the mail logs on the server so it will help you to solve the problemor you can paste the logs here.
The issue is only with the username and password.Please make sure that you are using the correct login details.Please provide us the mail logs so that we can help you more better.
Could be wrong outgoing mail server. If this is the case, search the name of your internet provider along with outgoing mail server. An indicator this is the problem is if you are recieving emails, but cant send.