how can I get cox.net email to work with linux? I can receive email. But when I send it I get this error

Sending failed:
The server did not accept the sender address.
The server responded: "Sender address is missing a domain "
The message will stay in the 'outbox' folder until you either fix the problem (e.g. a broken address) or remove the message from the 'outbox' folder.
Note: Other messages will also be blocked by this message, as long as it is in the 'outbox' folder
The following transport protocol was used:
smtp.west.cox.net

I am using Kmail in Mandrake 9.2 -- any help would be great

Weird. this is the second time I've seen this error ni kmail. Not too sure if it's a simple setup error or something deeper. The odd part is that the error message is saying that the 'following protocol was used' <smtp upsteream host>

You been messing with sendmail recently?

first time setting it up -- receives email fine - would another program work - what one should I use?