Apply suggestions from code review

Co-authored-by: Niklas van Schrick <mc.taucher2003@gmail.com>

Author: Knerio

app/mailers/application_mailer.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class ApplicationMailer < ActionMailer::Base
-  default from: Sagittarius::Configuration.config[:rails][:mailer][:username]
+  default from: Sagittarius::Configuration.config[:rails][:mailer][:from]
   layout 'mailer'
 
   def test_mail

config/sagittarius.example.yml

@@ -2,6 +2,7 @@ rails:
   threads: 3
   log_level: info
   mailer:
+    from: Code0 <testmail@code0.tech>
     address: smtp.example.com
     port: 587
     domain: code0.tech

lib/sagittarius/configuration.rb

@@ -27,6 +27,7 @@ def self.defaults
           },
           log_level: 'info',
           mailer: {
+            from: 'Code0 <testmail@code0.tech>',
             address: 'smtp.example.com',
             port: 587,
             domain: 'code0.tech',