Postfix SMTP auth not working with virtual mailboxes + SASL + Courier userdb

So I've read a variety of tutorials and how-to's and I'm struggling to make sense of how to get SMTP auth working with virtual mailboxes in Postfix. I used this Ubuntu tutorial to get set up. I'm using Courier-IMAP and POP3 for reading mail which seems to be working without issue.

However, the credentials used to read a mailbox are not working for SMTP. I can see from /var/log/auth.log that PAM is being used, does this require a UNIX user account to work? As I'm using virtual mailboxes to avoid creating user accounts.

li305-246 saslauthd[22856]: DEBUG: auth_pam: pam_authenticate failed: Authentication failure
li305-246 saslauthd[22856]: do_auth         : auth failure: [user=fred] [service=smtp] [realm=] [mech=pam] [reason=PAM auth error]

/var/log/mail.log

li305-246 postfix/smtpd[27091]: setting up TLS connection from mail-pb0-f43.google.com[209.85.160.43]
li305-246 postfix/smtpd[27091]: Anonymous TLS connection established from mail-pb0-f43.google.com[209.85.160.43]: TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits)
li305-246 postfix/smtpd[27091]: warning: SASL authentication failure: Password verification failed
li305-246 postfix/smtpd[27091]: warning: mail-pb0-f43.google.com[209.85.160.43]: SASL PLAIN authentication failed: authentication failure

I've created accounts in userdb as per this tutorial. Does Postfix also use authuserdb?

What debug information is needed to help diagnose my issue?

main.cf:

# TLS parameters
smtpd_tls_cert_file = /etc/ssl/certs/smtpd.crt
smtpd_tls_key_file = /etc/ssl/private/smtpd.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache

# SMTP parameters

smtpd_sasl_local_domain =
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_recipient_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination
smtp_tls_security_level = may
smtpd_tls_security_level = may
smtpd_tls_auth_only = no
smtp_tls_note_starttls_offer = yes
smtpd_tls_CAfile = /etc/ssl/certs/cacert.pem
smtpd_tls_loglevel = 1
smtpd_tls_received_header = yes
smtpd_tls_session_cache_timeout = 3600s
tls_random_source = dev:/dev/urandom

/etc/postfix/sasl/smtpd.conf

pwcheck_method: saslauthd
mech_list: plain login

/etc/default/saslauthd

START=yes

PWDIR="/var/spool/postfix/var/run/saslauthd"
PARAMS="-m ${PWDIR}"
PIDFILE="${PWDIR}/saslauthd.pid"
DESC="SASL Authentication Daemon"
NAME="saslauthd"
MECHANISMS="pam"
MECH_OPTIONS=""
THREADS=5
OPTIONS="-c -m /var/spool/postfix/var/run/saslauthd"

/etc/courier/authdaemonrc

authmodulelist="authuserdb"

I've only modified one line in authdaemonrc and restarted the service as per this tutorial. I've added accounts to /etc/courier/userdb via userdb and userdbpw and run makeuserdb as per the tutorial.

SOLVED

Thanks to Jenny D for suggesting use of rimap to auth against localhost IMAP server (which reads userdb credentials).

I updated /etc/default/saslauthd to start saslauthd correctly (this page was useful)

MECHANISMS="rimap"
MECH_OPTIONS="localhost"
THREADS=0
OPTIONS="-c -m /var/spool/postfix/var/run/saslauthd -r"

After doing this I got the following error in /var/log/auth.log:

li305-246 saslauthd[28093]: auth_rimap: unexpected response to auth request: * BYE [ALERT] Fatal error: Account's mailbox directory is not owned by the correct uid or gid: 
li305-246 saslauthd[28093]: do_auth         : auth failure: [user=fred] [service=smtp] [realm=] [mech=rimap] [reason=[ALERT] Unexpected response from remote authentication server]

This blog post detailed a solution by setting IMAP_MAILBOX_SANITY_CHECK=0 in /etc/courier/imapd.

Then restart your courier and saslauthd daemons for config changes to take effect.

sudo /etc/init.d/courier-imap restart
sudo /etc/init.d/courier-authdaemon restart
sudo /etc/init.d/saslauthd restart

Watch /var/log/auth.log while trying to send email. Hopefully you're good!