Error on sending email to Internal and external email

[Chris Morris]

I get
"The message could not be sent because one of the recipients was rejected by
the server. The rejected e-mail address was 'jcmorris@******.net'. Subject
'*TESTING4', Account: 'Rudd', Server: '192.168.10.2', Protocol: SMTP, Server
Response: '450 Mailbox unavailable.', Port: 25, Secure(SSL): No, Server
Error: 450, Error Number: 0x800CCC79"
When sending email to both a internal and external email address on the from
line. Is there somthing that I have turned on that could cause this in
sendmail? I can send to either intenal or external address with no problem,
but this one is strange to me. Thanks all for any help.   Chris Morris