How to send mail to mailbox with disabled domain?

[Kai Schaetzl]

Given the following:

mailboxes:
user1 at example1.com
user2 at example1.com
user3 at example1.com
etc.

aliases:
whatever at example1.com -> user1 at example1.com
whatever at example2.com -> user1 at example1.com
whatever at example3.com -> user1 at example1.com

Now the problem:
example1.com MX goes elsewhere (doesn't point to this server anymore).
Domains example2.com and example3.com still point to that server and 
should be able to accept mail.
I have to disable mail acceptance for example1.com. 
If not, mail sent *from* that server (e.g. from a web form) to that domain 
will not leave the server. 
However, if I disable example1.com for mail dovecot lmtp will not deliver 
mail to this mail box anymore, although the mailbox still exists.

How can I solve this? Is there a way of solving this, but keeping the 
domain example1.com in the name for these mailboxes?
Or is there a way to tell dovecot to ignore domains for mailbox names? 
e.g. deliver to "user1"? (All user localparts are unique.)

Thanks for any hints.

Kai