diff --git a/Gemfile b/Gemfile
index 7a070b3..797263e 100644
--- a/Gemfile
+++ b/Gemfile
@@ -20,3 +20,6 @@ if RUBY_VERSION.to_f > 2.4
   gem "rubocop",           "~> 0.92"
   gem "rubocop-packaging", "~> 0.5"
 end
+
+# Required for testing Rails 6.1 on MRI 3.1+
+gem "net-smtp" if RUBY_VERSION.to_f > 3.0