Hi,
the rejects to accept your mail for delivery because the server you're sending from (your web server) is not in a domain that is in the list of domains acceptable for relaying on the mail server.
If you send mail to receipients managed by that server it would work. But if you try to use that server to send mail to receipients outside of the domain of that server or the domains configured on that server it will reject to send the mail.
Example:
Your webserver is www.domain1.com
The Mailserver is mail.domain2.com
The Mail server allows relaying for
Domain2.com, Domain3.com and Domain4.com and you try to send a mail to user@domain5.com the mail server would refuse to do that. If your web server was e.g. www.domain2.com you would be able to send mail to other domains.