Sendmail from single to multi domain settings

[Erik van der Meulen <e.van.der.meulen@avondel.nl>]

I have used sendmail for an extended period of time with a UUCP feed for
a single domain. 
Now I have registered additional domains and I am running into some
difficulties with my configuration.
My server is: souterrain.avondel.nl and besides using avondel.nl as a
mail domain, I also need to use paces.nl and dices.nl. Every user on my
system has one username at one domain only.
The receiving end works well, but I cannot prevent that outgoing mail
gets avondel.nl as domain in the From: header. No matter what I put in
my mail client. I think I have tried every (combination of) option in
the sendmailconfig script, without reproducable results.
My /etc/usr/sendmail.mc currently looks like:

divert(0)
VERSIONID(`@(#)sendmail.mc	8.9.1a (Linux) 19981026')
OSTYPE(debian)dnl
LOCAL_CONFIG
FEATURE(masquerade_envelope)dnl
Cwavondel.nl
FEATURE(use_cw_file)dnl
FEATURE(use_ct_file)dnl
FEATURE(redirect)dnl
define(`confMAILER_NAME', `postmaster')dnl
define(`confTO_QUEUEWARN', `1d')dnl
MAILER_DEFINITIONS
MAILER(local)dnl
MAILER(smtp)dnl

LOCAL_CONFIG
MASQUERADE_AS(avondel.nl)dnl
## Custom configurations below (will be preserved)

FEATURE(`virtusertable', `hash -o /etc/mail/virtusertable.db')dnl
FEATURE(`genericstable', `hash -o /etc/mail/genericstable.db')dnl
GENERICS_DOMAIN_FILE(`/etc/mail/genericsdomain')dnl

define(`confMAX_MESSAGE_SIZE', `30000000')dnl
define(`UUCP_MAILER_MAX', `30000000')dnl

MAILER(uucp)dnl
FEATURE(`promiscuous_relay')dnl
define(`SMART_HOST', uucp-dom:netsystem)dnl

I am not sure if I understand the function of genericsdomain well; does
it contain domains of the client machines who's mail will be mapped, or
does it contain names of domains to which sendmail can map?

For clarity, I would like to give my users a fixed domain, irrespective
of what they put in their mail client, or if they are locally on my
machine, use pop or whatever. This domain should be connected to their
username/password with which they connect to my machine.

I hope this can be done. Any advice is very welcome, I have spent quite
a bit of time without success to date.

Thanks.

--
  Erik van der Meulen <e.van.der.meulen@avondel.nl>