diff --git a/mailbag.gemspec b/mailbag.gemspec
index 6970eeb..5fdd2aa 100644
--- a/mailbag.gemspec
+++ b/mailbag.gemspec
@@ -18,5 +18,5 @@ Gem::Specification.new do |s|
   s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
   s.require_paths = ["lib"]
 
-  s.add_dependency "actionmailer", "~> 3.2.1"
+  s.add_dependency "actionmailer", ">= 3.2.1", "< 6.1.0"
 end