diff --git a/src/lib-storage/mail-storage-service.c b/src/lib-storage/mail-storage-service.c
index b94d08b513..b347e7e157 100644
--- a/src/lib-storage/mail-storage-service.c
+++ b/src/lib-storage/mail-storage-service.c
@@ -1529,6 +1529,12 @@ mail_storage_service_all_iter_deinit(struct mail_storage_service_ctx *ctx)
 }
 
 void mail_storage_service_all_init(struct mail_storage_service_ctx *ctx)
+{
+	mail_storage_service_all_init_mask(ctx, "");
+}
+
+void mail_storage_service_all_init_mask(struct mail_storage_service_ctx *ctx,
+					const char *user_mask_hint)
 {
 	enum auth_master_flags flags = 0;
 
@@ -1542,7 +1548,8 @@ void mail_storage_service_all_init(struct mail_storage_service_ctx *ctx)
 		flags |= AUTH_MASTER_FLAG_DEBUG;
 	ctx->iter_conn = auth_master_init(auth_master_get_socket_path(ctx->conn),
 					  flags);
-	ctx->auth_list = auth_master_user_list_init(ctx->iter_conn, "", NULL);
+	ctx->auth_list = auth_master_user_list_init(ctx->iter_conn,
+						    user_mask_hint, NULL);
 }
 
 int mail_storage_service_all_next(struct mail_storage_service_ctx *ctx,
diff --git a/src/lib-storage/mail-storage-service.h b/src/lib-storage/mail-storage-service.h
index 65b633194d..603aeaccb7 100644
--- a/src/lib-storage/mail-storage-service.h
+++ b/src/lib-storage/mail-storage-service.h
@@ -111,6 +111,11 @@ int mail_storage_service_lookup_next(struct mail_storage_service_ctx *ctx,
 void mail_storage_service_user_free(struct mail_storage_service_user **user);
 /* Initialize iterating through all users. */
 void mail_storage_service_all_init(struct mail_storage_service_ctx *ctx);
+/* Same as mail_storage_service_all_init(), but give a user mask hint to the
+   userdb iteration lookup. This itself isn't yet guaranteed to filter out any
+   usernames. */
+void mail_storage_service_all_init_mask(struct mail_storage_service_ctx *ctx,
+					const char *user_mask_hint);
 /* Iterate through all usernames. Returns 1 if username was returned, 0 if
    there are no more users, -1 if error. */
 int mail_storage_service_all_next(struct mail_storage_service_ctx *ctx,