Sounds like you dont have specified your domains in : /etc/sendmail.cw
Each domain name should be added on a separate line in "/etc/sendmail.cw". You can do this with your favorite editor on UNIX (vi, pico, etc). Your sendmail.cw should look something like this:
# sendmail.cw - include all aliases for your machine here.localhost
mydomain1.com
mydomain2.com
And dont forget to restart your sendmail config.
Hope that helps.
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