[Dovecot] Disable imap proxy access

I work on a dovecot cluster (15 servers) we need to add a new storage array and move the data for a list of user from the old storage to the new one. We use dovecot proxy to redirect imap and pop3 to the same server everytime.

How to disable pop3 and imap access during the data migration process (duration 5 minutes to 10 minutes). Can i send a custom error message to our users ?

Thanks for your help

Dominique Feyer