clarification needed on working of a mail server using sendmail

Greetings All,

I have some confusion regarding working of a mail server . Please
help me in clarifying this point.
The scenario is like this.

There is a mail server running in our organisation for sending
mail ( sendmail ) . No POP and IMAP
service are running on this mail server and we use to check our
mails using webmail , a PHP based
webmail program.

I have configured EVOLUTION and in the sending mail option , I
have used SMTP as the server type
and in the server's ip address i have to the mail server.
I have not configured the receive mail section as there is no POP
and IMAP service running in our
network.

As far as my understanding goes , I should be able to send mail
using evolution as it is configured for
SMTP and the mail server is a working mail server . Now when I try
to send mail , I receive the following
error.

""
Error while performing operation.
MAIL FROM command failed: Access denied
""

Please clarify me why I am getting this error and using the above
configuration is it possible to send mail

regards
zaman