Nologin extra field
reason=System is being upgraded, please try again later.
Unfortunately many clients don't show the reason to the user at all and just assume that the password was given wrong, so it might not be a good idea to use this unless the system will be down for days and you don't have a better way to notify the users.
Note: if you want to entirely block the user from logging in (i.e. account is suspended), with no IMAP referral information provided, you must ensure that neither proxy or host are defined as one of the passdb extra fields. The order of preference is: proxy, host, then nologin.