Auto-respond to email sent to old address

[mathew]

I've migrated a small business away from their ISP email accounts to email accounts for their domain. I'm using getmail to seamlessly fetch emails from their ISP accounts and deliver the emails to the appropriate domain email account.

I would like to generate an auto-response when someone sends an email to one of the old ISP email addresses.

• Email message should still be delivered
• Same message to be sent for all ISP email addresses

Zimbra mail filters didn't appear to have an auto-response option. I could use a getmail filter and write a custom script to check the to header, but that feels more complex than it needs to be.