Is it possible to serve one domain with two SMTP/POP servers (main and backup)?

Question

Mail server

Hello.
The organization runs Microsoft Exchange 365 as its primary mail server. There is a need and desire to leave some of the accounts in the organization's domain, but to service them on another mail server (SMTP, POP / IMAP), such as Zimbra or Postfix / Dovecot. This is mainly dictated by the desire to save on licenses for devices, insignificant users (rare remote guests), etc.
Is it possible to somehow write a rule for the same domain: if the user is not found on Microsoft, then see the second server (link to the server or a DNS entry)? Is there such a thing and what is it called?
Thank you.