bare minimum sendmail config for full masquerading

Hello,

I'm trying to configure sendmail for a bare minimum job almost
like a nullmailer ... but I can't get the rewriting done ...

The configuration is for an appliance using CentOS/sendmail
and I'd like to be able to do the following:

+ no local mail on the appliance
+ all outgoing mail sshould be sent to a mailhub/smarthost
+ optionally authentication to the smarthost required
+ local aliasing is used to form 'mail-groups' for outgoing mail
+ the sender (envelope & header) of all outgoing mail should be
rewritten to a single email-adress for that box
(regardles which 'user' eg. process on that box created the
mail, the mail should always origin from thebox@example.com
- something like MASQUERADE_AS(`thebox@example.com')

I've been searching the web for a while and played with the
mc files, but I can't get the sender rewriting done.
Also the box is still doing dns-lookups for itself and the
target domain which are absolutely unneccessary if sending
to a smarthost ...

Any hints, ideas, tips very welcome

Many thanks

- Karl