If anyone can help with this I'd appreciate it. I'm trying to set up a
small intranet server, and want to send email out via PHP's mail command
and sendmail.

Now, in PINE and PHP, the system has set my sender email address to
"linuxuser@webserver.ourcompany.com". This results in some mailservers
dropping mail from our machine as "webserver.ourcompany.com" doesn't
resolve. The exact error message is "(reason: 550
<linuxuser@webserver.ourcompany.com>: Sender address rejected: Domain
not found)"

I guess I've made a configuration boo-boo when I set the box up, here's
what I have:

hostname = webserver
domainname = ourcompany.com
dnsdomainname = ourcompany.com

How do I configure my system so that the sender address is
"linuxuser@ourcompany.com" rather than it inserting the hostname in the
Sender: part? I'd prefer to do this at the system level rather than
modifying the Sender: address in each and every application I use.
(I've no idea if the error I'm making is a Slackware one, or a Sendmail
one).

Just a note: The intranet server is behind a firewall, our domain
'ourcompany.com' is valid, but the subdomain 'webserver.ourcompany.com'
is not. That's simply what we've called the system on our internal
network. Is it at this point I'm going wrong, and should I call it
something else internally?

Thanks in advance