Auth-worker, Unknown User

[Stephen R Guglielmo]

Hi list,

I'm running Debian Jessie (8). I have Postfix and Dovecot working
successfully. I attempted to setup LMTP in order to enable sieve
support, but it seems I was not successful. Dovecot version 2.2.13.

Postfix accepts an incoming test mail I sent, then tries to deliver it
to Dovecot via LMTP:

dovecot: lmtp(13493): Connect from local
dovecot: auth-worker(13495): pam(srg at example.com): unknown user
dovecot: lmtp(13493): Disconnect from local: Successful quit

Postfix then bounces my mail with a "550 User doesn't exist." It seems
that Dovecot is trying to lookup my whole address (srg at example.com) in
PAM instead of just the username ("srg"), which does exist. What can I
do to avoid this?

I have searched the internet about this and found a few references to
"args = %s" and "args = username_format=%n" for passdb and/or userdb,
but I have tried both (restarting Dovecot) to no avail.

Thank you!

`doveconf -n` below:

# 2.2.13: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-042stab103.6 x86_64 Debian 8.0 
auth_verbose = yes
first_valid_uid = 1000
last_valid_uid = 1000
listen = 167.114.96.243, 2607:5300:100:200::fce
lmtp_save_to_detail_mailbox = yes
mail_location = maildir:~/Maildir
namespace inbox {
  inbox = yes
  location = 
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix = 
}
passdb {
  driver = pam
}
plugin {
  recipient_delimiter = +
  sieve = ~/.dovecot.sieve
  sieve_dir = ~/sieve
}
protocols = " imap lmtp"
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0660
    user = postfix
  }
}
service imap-login {
  inet_listener imap {
    port = 143
  }
  inet_listener imaps {
    port = 0
  }
}
service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0600
    user = postfix
  }
}
ssl_cert = </etc/ssl/private/dovecot.cacert.pem
ssl_cipher_list = HIGH:!aNULL:!eNULL:!EXP:!SSLv2
ssl_dh_parameters_length = 2048
ssl_key = </etc/ssl/private/dovecot.privkey.pem
ssl_prefer_server_ciphers = yes
ssl_protocols = TLSv1 TLSv1.1 TLSv1.2
userdb {
  driver = passwd
}
protocol lmtp {
  mail_plugins = sieve
}
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