Is there a way to send custom warning to all IMAP users?

[francis picabia]

We have migrated all email on a server to a cloud email platform.
The users were notified by email beforehand, but hundreds are still
connecting to the standard IMAP service.  They may not
even remember they have set up devices to connect here.
Is there a way to send a custom warning through some setting,
similar to how quota warnings are generated.  Really if there is
any error I can fake, and customize the message, it would work.
We are using Linux, pam authentication, Cyrus with saslauthd.

Just shutting down the service is also a solution, but given over 600
unique users have logged in today, I'd rather not dump that load on
the service desk.