Hello everybody.
Apparently I am learning PHP, and my project is that I have to write a script which subscribes new user to a forum. I have static IP from cyberonic.com DSL and Apache web server running on this IP. I have Sendmail running as well and all users on the machine can send emails to each other which means that Sendmail works ok.
Now here is the problem. After a user fills out the subscription form and sends it to my web server, PHP script supposed to send a confirmation email to the user stating that all went ok. PHP uses Sendmail to do that. When I test the script, mail gets returned with the error message stating that I need a real domain name as a return address and not something like apache@localhost.
I am sure that I have correct return address in the header set up in PHP mail() function. But Sendmail uses apache@localhost as a return address which is not a legitimate email for Internet.
Here is what I have in hosts file
127.0.0.1 localhost
192.168.1.3 watchrush
When I test out the subscription form and put my email address that I have with my ISP (ajinian@cyberonic.com), this mail goes through with out any problems. But as soon as I use let's say my yahoo mail it comes back saying that the return address must have legitimate domain.
I did some research on the net and in the forum, but unfortunately could not fix the problem. If someone knows how to make Sendmail use my domain name instead of apache@localhost please let me know. Looks like I really need to get a book about how email works and Sendmail particularly.
Thank you all in advance for your replies.

Well, you most likely need a registered domain with MX records that is somewhat authentic. Most ISP's and mail servers these days put up barricades to prevent spammers from spamming their networks. If they didn't put these up, anyone could setup sendmail with any connection and no domain, MX record and so on and spam the living hell out of the world..

Thanks for telling me that trickykid :-)
Now the following question will be is it possible to resolve this problem with things that I already have?
And I have following:
registered domain which is www.watchrush.com (points to my web server IP)registered email sales@watchrush.com.
All of which are registered with networksolutions.com.
If this not enough then what else do I need in order to use sendmail and do not scare hell out of other mail servers :-)
Thank you so much for your reply.