Sendmail SMTP server issue

Hi,

I have configured sendmail SMTP server in Redhat Linux machine.
The linux box is using the ISPs DNS server to resolve the domain name.
We have a mail server hosted by third party. In our organisation we would leke to use the Sendmail SMTP Server to relay mails to outside world. As our ISP will not be providing their SMTP server to relay our mails. Hence we thought of creating a SMTP relay server using Sendmail in Redhat linux 7.
Now whenever I try to send mail through this machine it gives me this error “User Unknown”
Please if I need to attach the maillog file?

Please let me know what I should do to resolve this issue.

Regards,
Raja