If you receive the error 451 Greylisted, please try again in 60 seconds when sending an e-mail. The reason behind this error is, you have not configured authentication in your E-mail Application when sending an e-mail. Also, the mail server is treating your connection as an external mail server, not a local user.
SMTP authentication issue
Ensure that your E-mail application is using valid SMTP credentials such as sending an email with valid email address and password.