Forward undelivered email to on-premises SMTP server

I have a customer who wants to migrated only 30 of his 80 mail users to Office 365.  He wants to then configure the MX records to point to Office 365 and forward any undelivered email back down to the POP/SMTP server on-premises.  This type of functionality would be possible in On-Premises Exchange, but how do I  do it with Exchange Online?

Any help would really be appreciated.

Regards,

Gerhard