Reworking the internals of Devise::Mailer - mainly so headers can be tweaked without overwriting/rewriting #setup_mail. (I'm using a custom mailer which is a subclass of Devise::Mailer, and want to change headers/templates in some situations - but perhaps this is useful generally).

app/mailers/devise/mailer.rb

@@ -1,6 +1,6 @@
 class Devise::Mailer < ::ActionMailer::Base
   include Devise::Controllers::ScopedViews
-  attr_reader :devise_mapping, :resource
+  attr_reader :scope_name, :resource
 
   def confirmation_instructions(record)
     setup_mail(record, :confirmation_instructions)
@@ -13,24 +13,38 @@ class Devise::Mailer < ::ActionMailer::Base
   def unlock_instructions(record)
     setup_mail(record, :unlock_instructions)
   end
-
+  
   private
-
+  
   # Configure default email options
   def setup_mail(record, action)
-    @scope_name     = Devise::Mapping.find_scope!(record)
-    @devise_mapping = Devise.mappings[@scope_name]
-    @resource       = instance_variable_set("@#{@devise_mapping.name}", record)
-
+    initialize_from_record record
+    
+    mail headers_for(action)
+  end
+  
+  def initialize_from_record(record)
+    @scope_name = Devise::Mapping.find_scope!(record)
+    @resource   = instance_variable_set("@#{devise_mapping.name}", record)
+  end
+  
+  def devise_mapping
+    @devise_mapping ||= Devise.mappings[scope_name]
+  end
+  
+  def headers_for(action)
     headers = {
-      :subject => translate(@devise_mapping, action),
-      :from => mailer_sender(@devise_mapping),
-      :to => record.email,
+      :subject       => translate(devise_mapping, action),
+      :from          => mailer_sender(devise_mapping),
+      :to            => resource.email,
       :template_path => template_paths
     }
-
-    headers.merge!(record.headers_for(action)) if record.respond_to?(:headers_for)
-    mail(headers)
+    
+    if resource.respond_to?(:headers_for)
+      headers.merge!(resource.headers_for(action))
+    end
+    
+    headers
   end
 
   def mailer_sender(mapping)