This documentation is for Dovecot v2.x, see wiki1 for v1.x documentation.

Nologin extra field

User isn't allowed to log in even if the password matches. Commonly used with proxying and login referrals, but may also be used standalone. One way to use this would be perhaps:

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.