Solved, though the solution was not exactly what you suggested. I tried that
and had no luck; but a little more googling got me checking
/var/log/auth.log, where I found the following:

May 21 00:05:56 g4 postfix/smtpd[3295]: sql_select option missing
May 21 00:05:56 g4 postfix/smtpd[3295]: auxpropfunc error no mechanism
available
May 21 00:05:56 g4 postfix/smtpd[3295]: _sasl_plugin_load failed on
sasl_auxprop_plug_init for plugin: sql

A little more googling turned up a solution. The correct parameter is:

smtpd_sasl_path = smtpd

Found the answer here:
http://archives.neohapsis.com/archives/postfix/2007-04/1445.html

On 5/19/07, Josh Paetzel <josh at tcbug.org> wrote:
>
> Thomas Johnson wrote:
> > I am in the process of migrating my mail server from Ubuntu Dapper to
> Debian
> > Etch (Ubuntu dropped PPC support and I just got a G4 to play with, so
> > switching architectures too). For the most part everything has been as
> > painless as copying the appropriate configuration files and installing
> the
> > requisite packages. One thing I'm having trouble with is getting postfix
> to
> > authenticate users using sasl reading from a mysql backend when they
> send
> > mail (using squirrelmail to send).
> >
> > When I try to send from squirrelmail I get the following error:
> > Authentication failed
> > Server replied: 535 5.7.0 Error: authentication failed: authentication
> > failure
> >
> > Checking the log files says the same thing:
> > May 17 17:33:48 g4 postfix/smtpd[3286]: warning: SASL authentication
> > problem: unable to open Berkeley db /etc/sasldb2: No such file or
> directory
> > May 17 17:33:48 g4 postfix/smtpd[3286]: warning: SASL authentication
> > problem: unable to open Berkeley db /etc/sasldb2: No such file or
> directory
> >
> > I've been digging around the Internet looking for an answer all
> afternoon
> > and comparing the configuration between the two systems. The
> configuration
> > for the pieces of the mail system is almost identical (by virtue of
> having
> > copied the files directly w/ minimal modification). The configuration
> works
> > flawlessly on the old system; Squirrelmail authenticates with postfix,
> which
> > uses sasl to query mysql. Judging from the error messages that show up
> in
> > the mail logs, it makes me think that sasl is trying to authenticate
> against
> > its own (non-existent) user database rather than sql. The sasl bits of
> the
> > /etc/postfix/main.cf are as follows:
> >
> > # sasl stuff (per the howto)
> > smtpd_sasl_auth_enable = yes
> > broken_sasl_auth_clients = yes
> > # below line was previous working config, changed to see if there is any
> > effect.
> > #smtpd_sasl_path = /etc/postfix/sasl:/usr/lib/sasl2
> > smtpd_sasl_path = /etc/postfix/sasl
> > smtpd_sasl_security_options = noanonymous
> > smtpd_sasl_local_domain =
> >
> > /etc/postfix/sasl/smtpd.conf is the file that should be referenced by
> the
> > smtpd_sasl_path in main.cf. It's contents are as follows:
> >
> > pwcheck_method: auxprop
> > auxprop_plugin: sql
> > mech_list: plain login cram-md5 digest-md5
> > sql_engine: mysql
> > sql_hostnames: 127.0.0.1
> > sql_user: xyzuser
> > sql_passwd: xyzpass
> > sql_database: maildb
> > sql_select: select clear from users where id='%u@%r' and enabled = 1
> >
> > FWIW, Squirrelmail is attempting to authenticate with DIGEST-MD5. I
> believe
> > I have the required packages installed on the new system, the relevant
> > version numbers are:
> >
> > new: libsasl2-2 2.1.22
> > old: libsasl2 2.1.19
> >
> > new: libsasl2-modules-sql 2.1.22
> > old: libsasl2-modules-sql 2.1.19
> >
> > new: postfix 2.3.8
> > old: postfix 2.2.10
> >
> > If anyone has any thoughts on this, i would really love to hear them as
> I'd
> > really like to be able to send mail again.
> >
> > Cheers,
> >
> > Tom Johnson
> >
> >
>
> The name of the parameter is a bit misleading, but if you set it to
> /etc/postfix/sasl/smtpd.conf it will be much happier.
>
> --
> Thanks,
>
> Josh Paetzel
>
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