Relative home path not allowed - but how is this relative?

I just set up my server with MySQL support for authentication/authorization compatible with Postfix Admin.

Initially, I was getting the "Relative home directory paths not supported" message, and it's easy to see why - Postfix Admin stores the maildir as "$domain/$userpart@$domain".

But the directories in 10-mail.conf are set as

mail_location = maildir:/home/mailboxes/%d/%u mail_home = maildir:/home/mailboxes/%d/%u

...and the user_query has been revised to:

user_query = SELECT concat('/home/mailboxes/', maildir) as full_maildir FROM mailbox WHERE username = '%u';

...so how are any of these relative paths?