When you encounter the error “Login incorrect or username/password”, the first step is to check your Cpanel or Plesk of your hosting account to see if the email address is set up correctly. Otherwise, the email address may not have been created correctly. Next, check if you are using the correct username. Usually the email username is your full email address, i.e. your domain name. Using an incorrect username will surely result in unsuccessful or incorrect login.
Finally, determine if you’re using the correct password. The way to do this is to go to your Cpanel or Plesk and see the correct password for each email account created. It’s easy to get confused, especially if you’ve already created several email accounts. Another Internet SMTP problem for sending emails is when the “Server not found” error occurs. The first thing you need to do is check if you are connected to the internet. Accessing your SMTP obviously requires that you have an active internet connection.
If you’re using a newly registered domain name, keep in mind that it usually takes a day or two for the domain to propagate on the Internet. This means that pointing your email client to mail.yourdomain won’t work anyway. And yes, in that case you will get the “Server not found” error.
If you’ve determined that the above is not the cause of the error, check your email client settings to see if you’re using the correct SMTP servers. Usually the right SMTP server is mail.yourdomain. However, many ISPs now require you to use their own SMTP server. Check with your ISP’s tech support what their policy is.
Yet another internet SMTP problem for sending email is when you get an almost immediate response saying “Mailbox full/Over quota/Could not create output file”. This is clearly not your problem. It just means that your recipient’s mailbox is full and has exceeded its quota. There is nothing you can do about it but recommend it.