I'm looking to move my mail server from a sendmail/linux system to Zimbra (v8.0.2). I've set up my Zimbra server (Ubuntu 12.04 LTS) but I've hit a problem.

External name is company.com
Internal AD domain is company.local
Current Email server: oldmail.company.local
All incoming email arrives @ oldmail.company.local
User email address is as myname@company.com
New Zimbra server is mail.company.com
MX records are set up and appear to work fine.
Zimbra domain is company.com with aliases for company.local and mail.company.com
Outgoing email for both servers is direct. I don't route mail.company.com's outgoing email through oldmail.company.local.
If a user is moved to zimbra I set oldmail up to forward the email to username@mail.company.com
This all works fine for incoming email.
Zimbra outgoing to non company addresses is fine.
Outgoing to local addresses on the Zimbra box work fine.

However If a Zimbra user tries to send an email to a user still on oldmail, ie replys to somename@company.com, Zimbra rejects it as a bad address.

Can Zimbra be easily modified to forward these to oldmail? If not how should I configure these systems to allow a phased migration?