diff --git a/postmark.gemspec b/postmark.gemspec
index 4cbeb5c..cb2e779 100644
--- a/postmark.gemspec
+++ b/postmark.gemspec
@@ -34,6 +34,6 @@ Gem::Specification.new do |s|
 
   s.add_dependency "json"
 
-  s.add_development_dependency "mail"
+  s.add_development_dependency "mail", "~> 2.7.0" # https://github.com/mikel/mail/pull/1539
   s.add_development_dependency "rake"
 end