diff --git a/src/smtp/mod.rs b/src/smtp/mod.rs
index 101a12bed..9bdc69235 100644
--- a/src/smtp/mod.rs
+++ b/src/smtp/mod.rs
@@ -1,5 +1,7 @@
 //! # SMTP transport module
 
+#![forbid(clippy::indexing_slicing)]
+
 pub mod send;
 
 use std::time::{Duration, Instant};