Split up mailer classes into their own files.

test/rails_app/app/mailers/users/from_proc_mailer.rb

@@ -0,0 +1,3 @@
+class Users::FromProcMailer < Devise::Mailer
+  default from: proc { 'custom@example.com' }
+end

test/rails_app/app/mailers/users/mailer.rb

@@ -1,12 +1,3 @@
 class Users::Mailer < Devise::Mailer
   default from: 'custom@example.com'
 end
-
-class Users::ReplyToMailer < Devise::Mailer
-  default from: 'custom@example.com'
-  default reply_to: 'custom_reply_to@example.com'
-end
-
-class Users::FromProcMailer < Devise::Mailer
-  default from: proc { 'custom@example.com' }
-end

test/rails_app/app/mailers/users/reply_to_mailer.rb

@@ -0,0 +1,4 @@
+class Users::ReplyToMailer < Devise::Mailer
+  default from: 'custom@example.com'
+  default reply_to: 'custom_reply_to@example.com'
+end