diff -r -u3 imap-4.5_orig2/src/osdep/unix/maildir.c imap-4.5_rename/src/osdep/unix/maildir.c
--- imap-4.5_orig2/src/osdep/unix/maildir.c	Sat Jun 26 23:53:59 1999
+++ imap-4.5_rename/src/osdep/unix/maildir.c	Sun Jun 27 01:22:21 1999
@@ -1180,6 +1180,7 @@
 long maildir_rename (MAILSTREAM *stream,char *old,char *new)
 {
   char tmp[MAILTMPLEN],tmpnew[MAILTMPLEN];
+  int i;
 
 				/* old mailbox name must be valid */
   if (!maildir_isvalid (old,NIL)) {
@@ -1195,8 +1196,26 @@
     return NIL;
   }
 
+                /* get the filesnames to rename */
+  maildir_file (tmp,old);
+  i = strlen(tmp);
+  if ( i > 4 && tmp[i-4] == '/' && tmp[i-3] == 'c' && tmp[i-2] == 'u' && tmp[i-1] == 'r' )
+  	tmp[i-4] = 0;
+
+  maildir_file (tmpnew,new);
+  i = strlen(tmpnew);
+  if ( i > 4 && tmpnew[i-4] == '/' && tmpnew[i-3] == 'c' && tmpnew[i-2] == 'u' && tmpnew[i-1] == 'r' )
+  	tmpnew[i-4] = 0;
+
+				/* new filename must not exist */
+  if (access (tmpnew,F_OK) == 0) {
+    sprintf (tmp,"Can't rename to mailbox %s: destination already exists as a non-mailbox",new);
+    mm_log (tmp,ERROR);
+    return NIL;
+  }
+
 				/* try to rename the directory */
-  if (rename (maildir_file (tmp,old),maildir_file (tmpnew,new))) {
+  if (rename (tmp,tmpnew)) {
     sprintf (tmp,"Can't rename mailbox %s to %s: %s",old,new,strerror (errno));
     mm_log (tmp,ERROR);
     return NIL;