Murder with Virtual Domains - deliveries from postfix fail

[Dan Gaudette]

Hello,

I have Cyrus Murder 2.3.12p2 up and running (1 frontend, 1 backend, 1 
mupdate), however when I enable virtdomains I receive the following 
error on the frontend:   imapd/lmtp[]: verify_user 
(frontend.example.com!user.steve) failed: Mailbox does not exist

Everything else seems to work fine (IMAP, POP, creating/deleting 
mailboxes with domain administrators), but the frontend server fails 
with the above message when receiving the email addressed to 
steve at example.com from Postfix (separate machine). 

Postfix returns: <steve at frontend.example.com>: host 
xxx.xx.xx.xxx[xxx.xx.xx.xxx] said: 550-Mailbox unknown.  Either there is 
no mailbox associated with this 550-name or you do not have 
authorization to see it. 550 5.1.1 User unknown (in reply to RCPT TO 
command)

I've tried recreating the mailbox after virtdomains was enabled, but 
with the same result. 

Am I missing something in my Cyrus configuration, or is this an issue 
with Postfix?

Thanks in advance,
Dan

Backend config:

# /etc/imapd.conf

    configdirectory: /var/cyrus/imap
    partition-default: /var/cyrus/mail
    syslog_prefix: imapd
    admins: cyrus murd admin at foo.bar
    
    proxyservers: murd
   
   # settings for the mupdate (master) server   
   mupdate_config: standard
   mupdate_username: back
   mupdate_authname: back
   mupdate_password: back
   mupdate_server: xxx.xxx.xxx.xxx
   mupdate_port: 3905
   
   # allow user moves and renames
   allowusermoves: 1
   
   # virtual domain
   # allowed values: off, userid, on
   virtdomains: userid
   defaultdomain: example.com
   servername: backend

   guid_mode: sha1
   
    # Sieve
    sievedir: /var/cyrus/sieve
    sendmail: /usr/sbin/sendmail

    # Databases
    hashimapspool: true
    annotation_db: skiplist
    duplicate_db: skiplist
    mboxlist_db: skiplist
    quota_db: quotalegacy
    seenstate_db: skiplist
    subscription_db: skiplist

    # Authentication
    allowanonymouslogin: no
    allowplaintext: yes

    # Use saslauthd to authenticate users
    sasl_mech_list: plain
    sasl_pwcheck_method: saslauthd

    # Bounce messages that are over quota
    lmtp_over_quota_perm_failure: yes

    # tls/ssl support
    tls_cert_file: /var/cyrus/imap/tls/server.pem
    tls_key_file: /var/cyrus/imap/tls/server.pem
    tls_ca_file: /var/cyrus/imap/tls/server.pem


Frontend config is similar, but with proxy_authname and  backend_password