--- a/src/lib-storage/list/mailbox-list-index.c	Wed Nov 29 14:38:38 2017 +0200
+++ b/src/lib-storage/list/mailbox-list-index.c	Wed Nov 29 15:39:45 2017 +0200
@@ -587,9 +587,12 @@
 {
 	struct mailbox_list_index *ilist = INDEX_LIST_CONTEXT(list);
 	struct mail_storage *const *storagep;
+	enum mail_storage_list_index_rebuild_reason reason;
 	int ret = 0;
 
-	if (!ilist->call_corruption_callback)
+	if (ilist->call_corruption_callback)
+		reason = MAIL_STORAGE_LIST_INDEX_REBUILD_REASON_CORRUPTED;
+	else
 		return 0;
 
 	/* make sure we don't recurse */
@@ -599,6 +602,7 @@
 
 	array_foreach(&list->ns->all_storages, storagep) {
 		if ((*storagep)->v.list_index_corrupted != NULL) {
+			(*storagep)->list_index_rebuild_reason = reason;
 			if ((*storagep)->v.list_index_corrupted(*storagep) < 0)
 				ret = -1;
 			else {

--- a/src/lib-storage/mail-storage-private.h	Wed Nov 29 14:38:38 2017 +0200
+++ b/src/lib-storage/mail-storage-private.h	Wed Nov 29 15:39:45 2017 +0200
@@ -20,6 +20,11 @@
 
 #define MAIL_SHARED_STORAGE_NAME "shared"
 
+enum mail_storage_list_index_rebuild_reason {
+	/* Mailbox list index was found to be corrupted. */
+	MAIL_STORAGE_LIST_INDEX_REBUILD_REASON_CORRUPTED,
+};
+
 struct mail_storage_module_register {
 	unsigned int id;
 };
@@ -48,10 +53,12 @@
 					 const char *vname,
 					 enum mailbox_flags flags);
 	int (*purge)(struct mail_storage *storage);
-	/* Called when mailbox list index corruption has been detected.
+	/* Called when mailbox list index rebuild is requested. Additional
+	   parameter is in storage.list_index_rebuild_reason.
+
 	   The callback should add any missing mailboxes to the list index.
 	   Returns 0 on success, -1 on temporary failure that didn't properly
-	   fix the index. */
+	   rebuild the index. */
 	int (*list_index_corrupted)(struct mail_storage *storage);
 };
 
@@ -152,6 +159,10 @@
 	struct mail_storage_callbacks callbacks;
 	void *callback_context;
 
+	/* FIXME: remove in v2.3 - should be a parameter to
+	   list_index_corrupted() */
+	enum mail_storage_list_index_rebuild_reason list_index_rebuild_reason;
+
 	struct mail_binary_cache binary_cache;
 	/* Filled lazily by mailbox_attribute_*() when accessing shared
 	   attributes. */