Sendmail masquerade

I'm trying to configure sendmail masquerading and it seems like I'm having a problem with m4.

My main problem is that internally generated emails are showing up externally as originating from: internal_user@internal1.mydomain.com.
internal1.mydomain.com doesn't resolve publicly, nor should it.
I'm trying to setup my aix 5.3 box to simply send emails that show up as:

info@mydomain.com

and possibly

contactus@myotherdomain.com

as well.

I'm trying to add the following, to sendmail.mc

FEATURE(always_add_domain)dnl
FEATURE(`masquerade_envelope')dnl
MASQUERADE_AS(`mydomain.com')dnl
MASQUERADE_DOMAIN(`mydomain.com')dnl
MASQUERADE_DOMAIN(localhost)dnl
MASQUERADE_DOMAIN(localhost.localdomain)dnl

but when I run:

m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

the result that my cf file is overwritten with just the output of my .mc file and all the original contents of my sendmail.cf file are lost

$ cat /etc/mail/sendmail.cf
FEATURE(always_add_domain)FEATURE(masquerade_envelope)MASQUERADE_AS(mydomain.com)MASQUERADE_DOMAIN(mydomain.com)MASQUERADE_DOMAIN(localhost)MASQUERADE_DOMAIN(localhost.localdomain)

I have a backup copy of sendmail.mc, that I've restored.

should m4 just basically be doing cat on my .mc file into my .cf file?

Any help or direction would be great. Thanks

You must place your sendmail.mc in the sendmail build directory.
Howto configure AIX sendmail with .mc files and m4 sais this is
/usr/samples/tcpip/sendmail/cf
Also please look for a README in /usr/samples/tcpip/sendmail