Maildir++ is the most commonly used quota backend with Maildir format. Note that Maildir++ quota works only with Maildir format. With other mailbox formats you should use dict quota with file backend. Dovecot implements the standard Maildir++ specification, so it's compatible with Courier, maildrop, Exim, etc.
There are two ways to configure Maildir++ quota limits:
Configure the limits in Dovecot. You most likely want to do this. See quota main page for how to do this configuration.
Make Dovecot get the limits from existing maildirsize files.
Only Maildir++-specific settings are described below. See Quota for more generic configuration.
Maildir++ quota relies on maildirsize file having correct information, so if your users can modify the file in some way (e.g. shell access), you're relying on the goodwill of your users for the quota to work.
You can't rely on Dovecot noticing external changes to Maildir and updating maildirsize accordingly. This happens eventually when quota is being recalculated, but it may take a while. Quota recalculation also currently doesn't trigger quota warning executions.
The maildirsize file in the Maildir root directory contains both the quota limit information and the current quota status. It contains a header in format:
<storage limit in bytes>S,<messages limit>C
If you don't configure any quota limits in Dovecot (quota=maildir with no other settings), Dovecot takes the limits from the header. If the file does not exist, quota isn't enforced.
If you configure quota limits in Dovecot, Dovecot makes sure that this header is kept up to date. If the file does not exist, it's simply rebuilt.
Once the maildirsize reaches 5120 bytes, the quota is recalculated and the file is recreated. This makes sure that if quota happens to be broken (e.g. externally deleted files) it won't stay that way forever.