diff -Naur cyrus-imapd-2.4.10.orig/imap/global.c cyrus-imapd-2.4.10/imap/global.c
--- cyrus-imapd-2.4.10.orig/imap/global.c	2011-07-04 17:05:01.000000000 +0200
+++ cyrus-imapd-2.4.10/imap/global.c	2011-08-04 08:21:17.000000000 +0200
@@ -392,6 +392,14 @@
     char *domain = NULL;
     int len = strlen(user);
     char buf[81];
+    char tmp[MAX_MAILBOX_NAME+1];
+
+    if (config_getstring(IMAPOPT_APPENDDOMAIN)) {
+        if( !strchr(user, '@') ) {
+            snprintf(tmp, (MAX_MAILBOX_NAME+1), "%s@%s", user, config_getstring(IMAPOPT_APPENDDOMAIN));
+            user = tmp;
+        }
+    }
 
     /* check for domain */
     if (config_virtdomains &&
diff -Naur cyrus-imapd-2.4.10.orig/lib/imapoptions cyrus-imapd-2.4.10/lib/imapoptions
--- cyrus-imapd-2.4.10.orig/lib/imapoptions	2011-07-04 17:05:01.000000000 +0200
+++ cyrus-imapd-2.4.10/lib/imapoptions	2011-08-04 08:21:17.000000000 +0200
@@ -220,6 +220,10 @@
    file or mailboxes list entry?  It's noisy so disabled by default, but
    can be very useful for tracking down what happened if things look strange */
 
+{ "appenddomain", NULL, STRING }
+/* Append this domain name to user names without a domain portion. Do not use
+   with the default domain option. */
+
 { "auth_mech", "unix", STRINGLIST("unix", "pts", "krb", "krb5")}
 /* The authorization mechanism to use. */
 
@@ -270,7 +274,10 @@
    mailbox that does not have a parent mailbox. */
 
 { "defaultdomain", NULL, STRING }
-/* The default domain for virtual domain support */
+/* The default domain for virtual domain support. If the domain part of
+   user@domain is the same as the value specified, it will be removed from
+   usernames. Use this option if you add virtual domain support and do not
+   want to rename exisiting mailboxes. See \fIappenddomain\fR. */
 
 { "defaultpartition", NULL, STRING }
 /* The partition name used by default for new mailboxes.  If not