The message specified here is passed on to a user who goes over quota. Note, that Dovecot can read the quota-exceeded message from a file. The second example below shows this usage:
quota_exceeded_message = Quota exceeded. quota_exceeded_message = </path/to/quota_exceeded_message.txt
With this setting, virtual sizes rather than physical sizes are used for quota-related calculations. The value yes is required here if you wish to use the quota back end called count.