[courier-users] Changing from userx@domain1.com to userx@domain2.com. Is it possible?

[f. U. al d. <fr...@gm...>]

Hello all,
I have a courier installation for many email domains. This has been working
fine for more than two years.
Now I face a situation where I need to accept emails like 'us...@do...'
that have to be rewritten to 'us...@do...' and sent to a second SMTP
server.
domain1.com is a local domain and I have a esmtproutes to get to domain2.com
.

I can't find a way to achieve this with the aliases, the only thing I see is
a virtual domain and all emails for that virtual domain going to the same
mailbox.
Is there a way to make an alias for a domain into a different domain without
changing the username?
Any ideas?