SMTP talking

[rouse01]

All this worked fine up to a few weeks ago...
What is the difference between how a windows mail client relaying thru my sendmail server talks to smtp and the "mail" program on my linux server talks to smtp? My windows clients incoming POP and outgoing SMTP both point to my linux sendmail server & work fine (I use fetchmail to pop mail from my isp). However I can't use "pine" or "mail" directly from my server to send email. A bounce message looks like this:
The original message was received at Sat, 20 Sep 2003 23:00:59 -0500
from root@localhost

----- The following addresses had permanent fatal errors -----
<stewartkc@someotheraddr.com>
(reason: 550 [PERMFAIL] someotheraddr.com requires valid sender)

----- Transcript of session follows -----
... while talking to smtp.myisp.net.:
>>> RCPT To:<stewartkc@someotheraddr.com>
<<< 550 [PERMFAIL] someotheraddr.com requires valid sender
550 5.1.1 <stewartkc@someotheraddr.com>... User unknown

I need to be able to send email from a PHP web application & believe that PHP is using SMTP for the mail feature. Also, my auto replier is bouncing now. So, don't all these go through sendmail, including emails from my windows clients?
I tried to specify username and password in the MTA config using: AuthInfo:smtp.my.isp &quot;U:validuser&quot; &quot;P:userpasswd&quot; &quot;M:CRAM-MD5&quot;

Any ideas appreciated.
Keith