sendmail and mailing list ?

okay here's my first real question problem for this forum

details of my server:
red hat 6.2/apache
110% relying on using webmin control panel to do this.

i installed a mailing list script on one of my domains on my server and when i tried to send a test mailing out to a test list of subscribers (actually just my own email addresses) it returned errors on different domains

[email protected] returned this error - Deferred: 451 ...
Sender domain must exist

[email protected] returned this error -
----- The following addresses had permanent fatal errors -----
[email protected]

----- Transcript of session follows -----
550 [email protected] ... Host unknown (Name server:
inca.outblaze.com.:
host not found)

for

[email protected]
[email protected]
[email protected]
[email protected]

which are all my email address, there were no problems and i
received the test mailing letter...

out of all those domains the following actually reside on the server with sendmail

anime2001.com
dragonball-search.com

how can i fix this ? anyone know what to do in webmin's sendmail area ?