Can someone help me understand how to setup aliases?

I have setup the incoming mail server (Settings/General Settings/Configure incoming email gateway) as follows:

Server type: IMAP

Server name: mail.mydomain.com.au

Port: 993

SSL/TLS: ticked

Username: contact@mydomain.com.au

Password: the same password that I can use to access the imap account from a dedicated email client (Apple Mail.app for example).

I try to send an email from Apple Mail (johnsmith@otherdomain.com.au) to contact@mydomain.com.au but when I click the 'Fetch now' button I see the following in the log:

2013-04-03 09:31:28,564 880 INFO rgerp openerp.addons.fetchmail.fetchmail: start checking for new emails on imap server RG mail

2013-04-03 09:31:29,077 880 ERROR rgerp openerp.addons.fetchmail.fetchmail: Failed to fetch mail from imap server RG mail.

Traceback (most recent call last):

File "/opt/openerp70/server/openerp/addons/fetchmail/fetchmail.py", line 205, in fetch_mail context=context)

File "/opt/openerp70/server/openerp/addons/mail/mail_thread.py", line 615, in message_process context=context)

File "/opt/openerp70/server/openerp/addons/mail/mail_thread.py", line 542, in message_route "Create an appropriate mail.alias or force the destination model." % (email_from, email_to, message_id)

AssertionError: No possible route found for incoming message from johnsmith@otherdomain.com.au to contact@mydomain.com.au (Message-Id <0A990467-8E67-4244-87E3-4EBBF0E58112 @mydomain.com.au>:).Create an appropriate mail.alias or force the destination model.

It seems to suggest I need to create an alias for that email to be routed to (for example) userx@mydomain.com.au.

Can anyone give me instructions on how to do this?