Explanation
The Dovecot configuration variable that specifies the location of user mail is mail_location. This variable defines the format and path of the mailboxes, and it can be overridden by userdb or namespace settings. The format of the mail_location specification is:
<mailbox-format>:<path>[:<key>=<value>[:<key2>=<value2>...]]
where <mailbox-format> is one of the supported mailbox formats, such as mbox, maildir, or dbox; <path> is the absolute path to the mail directory; and <key>=<value> are optional parameters that modify the behavior of the mailbox format.
For example, a mail_location setting for maildir format could look like this:
mail_location = maildir:~/Maildir
This means that the user's mail is stored in the Maildir subdirectory of their home directory.
References: You can find more information about the mail_location variable and its parameters in the following resources:
Mail Location Settings - Dovecot documentation
Config Variables - Dovecot documentation