Actually, I install SQLServer2008R2 developer edition. At that time I configured database mail it works fine to me. After I uninstalled and and re-install the instance again. But this time when I tried to configured the database mail again, the mail is not going. When I see database mail log there is the following message

The mail couldn't send to the recipients because of the mail server failed. Can't send mails to mail server. The SMTP server requires a secure connection or the client was not authenticated. The server response was 5.5.1. Authentication required.