diff --git a/conf/dovecot-checkpassword.py b/conf/dovecot-checkpassword.py
new file mode 100644
index 00000000..91979ec3
--- /dev/null
+++ b/conf/dovecot-checkpassword.py
@@ -0,0 +1,53 @@
+#!/usr/bin/python3
+#
+# This script implement's Dovecot's checkpassword authentication mechanism:
+# http://wiki2.dovecot.org/AuthDatabase/CheckPassword?action=show&redirect=PasswordDatabase%2FCheckPassword
+#
+# This allows us to perform our own password validation, such as for two-factor authentication,
+# which Dovecot does not have any native support for.
+#
+# We will issue an HTTP request to our management server to perform authentication.
+
+import sys, os, urllib.request, base64, json, traceback
+
+try:
+	# Read fd 3 which provides the username and password separated
+	# by NULLs and two other undocumented/empty fields.
+	creds = b''
+	while True:
+		b = os.read(3, 1024)
+		if len(b) == 0: break
+		creds += b
+	email, pw, dummy, dummy = creds.split(b'\x00')
+
+	# Call the management server's "/me" method with the
+	# provided credentials
+	req = urllib.request.Request('http://127.0.0.1:10222/me')
+	req.add_header(b'Authorization', b'Basic ' + base64.b64encode(email + b':' + pw))
+	response = urllib.request.urlopen(req)
+
+	# The response is always success and always a JSON object
+	# indicating the authentication result.
+	resp = response.read().decode('utf8')
+	resp = json.loads(resp)
+	if not isinstance(resp, dict): raise ValueError("Response is not a JSON object.")
+
+except:
+	# Handle all exceptions. Print what happens (ends up in syslog, thanks
+	# to dovecot) and return an exit status that indicates temporary failure,
+	# which is passed on to the authenticating client.
+	traceback.print_exc()
+	print(json.dumps(dict(os.environ), indent=2), file=sys.stderr)
+	sys.exit(111)
+
+if resp.get('status') != 'authorized':
+	# Indicates login failure.
+	# (sys.exit should not be inside the try block.)
+	sys.exit(1)
+
+# Signal ok by executing the indicated process, per the Dovecot
+# protocol. (Note that the second parameter is the 0th argument
+# to the called process, which is required and is typically the
+# file itself.)
+os.execl(sys.argv[1], sys.argv[1])
+
diff --git a/setup/mail-users.sh b/setup/mail-users.sh
index 387ce698..408389a6 100755
--- a/setup/mail-users.sh
+++ b/setup/mail-users.sh
@@ -26,31 +26,55 @@ fi
 
 # ### User Authentication
 
-# Have Dovecot query our database, and not system users, for authentication.
-sed -i "s/#*\(\!include auth-system.conf.ext\)/#\1/"  /etc/dovecot/conf.d/10-auth.conf
-sed -i "s/#\(\!include auth-sql.conf.ext\)/\1/"  /etc/dovecot/conf.d/10-auth.conf
+# Disable all of the built-in authentication mechanisms. (We formerly uncommented
+# a line to include auth-sql.conf.ext but we no longer use that.)
+sed -i "s/#*\(\!include auth-.*.conf.ext\)/#\1/"  /etc/dovecot/conf.d/10-auth.conf
 
-# Specify how the database is to be queried for user authentication (passdb)
-# and where user mailboxes are stored (userdb).
-cat > /etc/dovecot/conf.d/auth-sql.conf.ext << EOF;
+# Legacy: Delete our old sql conf files.
+rm -f /etc/dovecot/conf.d/auth-sql.conf.ext /etc/dovecot/dovecot-sql.conf.ext
+
+# Specify how Dovecot should perform user authentication (passdb) and how it knows
+# where user mailboxes are stored (userdb).
+#
+# For passwords, we would normally have Dovecot query our mail user database
+# directly. The way to do that is commented out below. Instead, in order to
+# provide our own authentication framework so we can handle two-factor auth,
+# we will use a custom system that hooks into the Mail-in-a-Box management daemon.
+#
+# The user part of this is standard. The mailbox path and Unix system user are the
+# same for all mail users, modulo string substitution for the mailbox path that
+# Dovecot handles.
+cat > /etc/dovecot/conf.d/10-auth-mailinabox.conf << EOF;
 passdb {
-  driver = sql
-  args = /etc/dovecot/dovecot-sql.conf.ext
+  driver = checkpassword
+  args = /usr/local/bin/dovecot-checkpassword
 }
 userdb {
   driver = static
   args = uid=mail gid=mail home=$STORAGE_ROOT/mail/mailboxes/%d/%n
 }
 EOF
+chmod 0600 /etc/dovecot/conf.d/10-auth-mailinabox.conf
 
-# Configure the SQL to query for a user's password.
-cat > /etc/dovecot/dovecot-sql.conf.ext << EOF;
-driver = sqlite
-connect = $db_path
-default_pass_scheme = SHA512-CRYPT
-password_query = SELECT email as user, password FROM users WHERE email='%u';
-EOF
-chmod 0600 /etc/dovecot/dovecot-sql.conf.ext # per Dovecot instructions
+# Copy dovecot-checkpassword into place.
+cp conf/dovecot-checkpassword.py /usr/local/bin/dovecot-checkpassword
+chown dovecot.dovecot /usr/local/bin/dovecot-checkpassword
+chmod 700 /usr/local/bin/dovecot-checkpassword
+
+# If we were having Dovecot query our database directly, which we did
+# originally, `/etc/dovecot/conf.d/10-auth-mailinabox.conf` would say:
+#
+#     passdb {
+#       driver = sql
+#       args = /etc/dovecot/dovecot-sql.conf.ext
+#     }
+#
+# and then `/etc/dovecot/dovecot-sql.conf.ext` (chmod 0600) would contain:
+#
+#    driver = sqlite
+#    connect = $db_path
+#    default_pass_scheme = SHA512-CRYPT
+#    password_query = SELECT email as user, password FROM users WHERE email='%u';
 
 # Have Dovecot provide an authorization service that Postfix can access & use.
 cat > /etc/dovecot/conf.d/99-local-auth.conf << EOF;