app/mailers/application_mailer.rb

@@ -3,6 +3,16 @@
 # frozen_string_literal: true
 
 class ApplicationMailer < ActionMailer::Base
-  default from: 'from@example.com'
+  class << self
+    private
+
+    def default_from
+      File.read('/run/secrets/smtp_user_name').strip
+    rescue Errno::ENOENT
+      'development@example.com'
+    end
+  end
+
+  default from: default_from
   layout 'mailer'
 end