Sendmail Access.db help

[Yang Xiao <yxiao2004 gmail com>]

Hello,
I'm trying to prevent anyone from spoofing incoming mail as internal
users. Example, my domain is mydomain.com
I don't want to anyone to connection to my mail gateway and send
emails as user mydomain com from the outside.

I thought this can be done by access table and it's working. but the
problem now is that it's rejecting local mail that it's trying to
forward to the mailhost as well, how can I fix this?

#access table
# Check the /usr/share/doc/sendmail/README.cf file for a description
# of the format of this file. (search for access_db in that file)
# The /usr/share/doc/sendmail/README.cf is part of the sendmail-doc
# package.
#
# by default we allow relaying from localhost...
127.0.0.1               OK
localhost               OK
From:localhost          OK
To:localhost            OK
From:mydomain.com           ERROR:"550 Everything that has a
beginning, has an end."


Many thanks,

Yang