If your mail for both domains is sent directly to your server then you can do the following:
Setup a separate user account to handle the
[email protected] addresses and turn on out-of-office to tell them of the change of email address and that the message has automatically been forwarded, auto-forward the email from
[email protected] to
[email protected], then you will have your solution wrapped up.
If you get spammers sending mail to the old address, then there will be some rogue messages trying to be sent, but such is life.