diff -Nuar samba-3.5.4~dfsg.orig/source3/passdb/lookup_sid.c samba-3.5.4~dfsg/source3/passdb/lookup_sid.c
--- samba-3.5.4~dfsg.orig/source3/passdb/lookup_sid.c	2010-09-15 12:40:20.000000000 +0200
+++ samba-3.5.4~dfsg/source3/passdb/lookup_sid.c	2010-09-15 12:42:42.000000000 +0200
@@ -55,8 +55,15 @@
 					PTR_DIFF(p, full_name));
 		name = talloc_strdup(tmp_ctx, p+1);
 	} else {
-		domain = talloc_strdup(tmp_ctx, "");
-		name = talloc_strdup(tmp_ctx, full_name);
+		p = strchr_m(full_name, '@');
+		if (p != NULL) {
+			name = talloc_strndup(tmp_ctx, full_name,
+						PTR_DIFF(p, full_name));
+			domain = talloc_strdup(tmp_ctx, p+1);
+		} else {
+			domain = talloc_strdup(tmp_ctx, "");
+			name = talloc_strdup(tmp_ctx, full_name);
+		}
 	}
 
 	if ((domain == NULL) || (name == NULL)) {