It could be that you've set up your e-mail client incorrectly. But if it was workiing previously, and you've not made any changes, it might be a problem with your e-mail provider. Before trying anything else, I would contact the provider and find out if they have a problem.
Your problem is just about entering your user name and password. In fact, it simple as just checking the checkbox that says "use the same username and password" for outgoing email. You notice that when you are using outlook or any mail client like that. In case you are using outlook, see to it that your outlook has username and password authentication for outgoing emails.