qpopper vs sendmail?

[Keith Osburn <kosburn tcboe telfair k12 ga us>]

I have Redhat 5.1. Sendmail is installed and working okay (or so it seems). Additionally, I have qpopper 2.53 installed for POP.  Here's my problem (or lack of knowledge). When connecting to my linux box via  dialup I can receive mail (I'm using Outlook -- but have tried others) but can send any out.  I get a message stating the receipt is not acceptable to your SMTP.  Using Pine on the Linux box I can send and receive. When I telnet into 110 the POP responds. 

Any help would be appreciated.