diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index 8fe5868d529ef..2eef57f7adcd0 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -446,6 +446,33 @@ def process(*args) #:nodoc:
       super
     end
 
+    class DeprecatedHeaderProxy < ActiveSupport::BasicObject
+      def initialize(message)
+        @message = message
+      end
+
+      def []=(key, value)
+        unless value.is_a?(String)
+          ActiveSupport::Deprecation.warn("Using a non-String object for a header's value is deprecated. " \
+            "You specified #{value.inspect} (a #{value.class}) for #{key}", caller)
+
+          value = value.to_s
+        end
+
+        @message[key] = value
+      end
+
+      def headers(hash = {})
+        hash.each_pair do |k,v|
+          self[k] = v
+        end
+      end
+
+      def method_missing(meth, *args, &block)
+        @message.send(meth, *args, &block)
+      end
+    end
+
     # Allows you to pass random and unusual headers to the new +Mail::Message+ object
     # which will add them to itself.
     #
@@ -462,9 +489,9 @@ def process(*args) #:nodoc:
     #   X-Special-Domain-Specific-Header: SecretValue
     def headers(args=nil)
       if args
-        @_message.headers(args)
+        DeprecatedHeaderProxy.new(@_message).headers(args)
       else
-        @_message
+        DeprecatedHeaderProxy.new(@_message)
       end
     end
 
diff --git a/actionmailer/test/base_test.rb b/actionmailer/test/base_test.rb
index ef3808ee65fd4..7ae0b98eecd13 100644
--- a/actionmailer/test/base_test.rb
+++ b/actionmailer/test/base_test.rb
@@ -76,6 +76,11 @@ def teardown
     assert_equal("Not SPAM", email['X-SPAM'].decoded)
   end
 
+  test "deprecated non-String custom headers" do
+    email = assert_deprecated { BaseMailer.welcome_with_fixnum_header }
+    assert_equal("2", email['X-SPAM-COUNT'].decoded)
+  end
+
   test "can pass random headers in as a hash to mail" do
     hash = {'X-Special-Domain-Specific-Header' => "SecretValue",
             'In-Reply-To' => '1234@mikel.me.com' }
diff --git a/actionmailer/test/mailers/base_mailer.rb b/actionmailer/test/mailers/base_mailer.rb
index 4f61e6978adfd..f11171875b77f 100644
--- a/actionmailer/test/mailers/base_mailer.rb
+++ b/actionmailer/test/mailers/base_mailer.rb
@@ -10,6 +10,11 @@ def welcome(hash = {})
     mail({:subject => "The first email on new API!"}.merge!(hash))
   end
 
+  def welcome_with_fixnum_header(hash = {})
+    headers['X-SPAM-COUNT'] = 2
+    mail({:template_name => "welcome", :subject => "The first email on new API!"}.merge!(hash))
+  end
+
   def welcome_with_headers(hash = {})
     headers hash
     mail