[exim] Struggling with SQL authenticator

Hello

I'm trying to write a plain text authenticator that will pick up a
username and password from a postgres database. The config I am working
with is:

SQL_AUTH_USERNAME = SELECT delivery_username FROM domains WHERE
domainname=lower('${quote_pgsql:$domain}')
SQL_AUTH_PASSWORD = SELECT delivery_password FROM domains WHERE
domainname=lower('${quote_pgsql:$domain}')

plain:
    driver = plaintext
    public_name = PLAIN
    client_send = ^${lookup pgsql{SQL_AUTH_USERNAME}}^${lookup
pgsql{SQL_AUTH_PASSWORD}}

Attempting to send a mail via this mechanism results in:

2015-02-21 13:07:27 1YP9Y0-0007Oe-T9 == martin@???
R=zomg_routed T=remote_smtp defer (0): Temporary internal error

I'm struggling to find out what the underlying problem is here, and
wondered if anyone on the list has some insight.

Thanks

Martin A. Brooks