diff --git a/Cargo.toml b/Cargo.toml
index d3d00c1..9918675 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -29,7 +29,7 @@ rand = { version = "0.7", optional = true }
 quoted_printable = { version = "0.4", optional = true }
 base64 = { version = "0.13", optional = true }
 once_cell = "1"
-regex = "1"
+regex = { version = "1", default-features = false, features = ["std"] }
 
 # file transport
 serde = { version = "1", optional = true, features = ["derive"] }