I am getting the below error When send mail to external domain
"The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account 1 (2010-07-07T14:59:26). Exception Message: Cannot send mails to mail server. (Syntax error, command unrecognized. The server response was: firstname.lastname@example.org relaying prohibited. You should authenticate first).
But if I send mail to my domain then they are able to receive the mail.
Can any one help me to solve the issue.