2. Build a new MYSQL_SELECT_CLAUSE upon the Query authdaemon issues at the moment with proper quotations.

Try this

MYSQL_SELECT_CLAUSE SELECT email, password, '', 5000, 5000, \ '/home/vmail/$(domain)/$(local_part)/', '', '', '', '' \ FROM users WHERE email = '$(local_part)@$(domain)'

I went right to trying this option because I'm fairly sure now that it doesn't process these variables anywhere other than the custom select field. And, while the query forms correctly (see below), I still can't log in. The query forms, the fields show up as they're supposed to, and mysql doesn't complain.

I wonder why the AUXOPTIONS-Field is not evaluated. I left it out because I think you can debug that if the other things are running.

I'm probably not even using it. I was aiming just to get the base query working before I started getting into specifics.

An important note for debugging: run the mysql command line interpreter with the same login credentials as used in authmysqlrc and run the mysql query from the logfile in it when you encounter errors. MySQL will tell you what's wrong or give you the same results that authdaemon gets and you'll propably see what's wrong.