The problem is I was able to see with Doctor Connection that Both POP and SMTP passed with green color.
But I am only able to receive mail. I can’t send mail. Whenever I want to send mail it has a error.
How to solve this problem? Is my server name is correct?
The server name should be fine, as long as it resolves to your server’s IP address.
You may want to make sure that your email client is configured to authenticate for outgoing SMTP messages. Not all email clients will do that by default.
If that doesn’t help – what error are you seeing? And if you look in the error logs in /var/log/maillog or /var/log/mail.log, what errors show up in the mail logs when that happens?