Trying to configure sendmail in Debian

I'm trying to configure sendmail on our Debian Linux server to send mail, using our Exchange server as a relay. I'm a total noob at all of this.

We can send mails to internal addresses (for example, if our domain name is ourdomain.com, the server can send mail to URL) with no problem.

We can't send mail to external addresses like URL or URL. Here's an example of what I get in /var/log/mail.log

Mar 7 14:45:29 newton sm-mta[17159]: r27JjOw1017159: to=<user9999@gmail.com>, ctladdr=<root@newton.forrest> (0/0), delay=00:00:05, xdelay=00:00:05, mailer=relay, pri=30455, relay=webmail.ourdomain.ws. [xxx.xxx.xxx.xxx], dsn=5.7.1, stat=User unknown

Also, I get this:



Does that mean the URL is unknown to newton, or unknown to Exchange?

Is there something in Exchange we need to configure?

I have no idea what to configure. Please help. Thanks!

Have you configured DNS correctly. Is your MX record pointing to correct server?

Paste the output of
cat /etc/sendmail.cf , /etc/mail/relay-domains and /etc/mail/access