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

[Roger Klorese]

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?