How to configure sendmail

[Olivier Nicole]

Hi,

I have this stupide little configuration that I cannot manage to get
working.

I have one machine a.domain.net that I want to be able to deliver
system mail (like cron and so on) with the following rules:

- user1 on a.domain.net has the same username as on domain.net; I
  want that mail sent to user1 is delivered to user1 at domain.net;

- user2 on a.domain.net has no corresponding user on domain.net, but
  it has an alias defined; I want to mail sent to user2 is delivered
  to the alias.

- of course, mail addressed to a full address x at y.z should be
  delivered accordingly, eventually using a mail relay.

I tried using masquerade in submit.mc, user1 is then rewritten as
user1.domain.net, but the alias for user2 is not parsed and user2 is
also rewritten as user2 at domain.net.

How can I solve my problem?

TIA,
Olivier