refactor: refactor the SMTP sender

Gemfile.lock

@@ -132,7 +132,7 @@ GEM
       rails-dom-testing (>= 1, < 3)
       railties (>= 4.2.0)
       thor (>= 0.14, < 2.0)
-    json (2.6.3)
+    json (2.7.1)
     kaminari (1.2.2)
       activesupport (>= 4.1.0)
       kaminari-actionview (= 1.2.2)
@@ -145,7 +145,7 @@ GEM
       activerecord
       kaminari-core (= 1.2.2)
     kaminari-core (1.2.2)
-    klogger-logger (1.3.2)
+    klogger-logger (1.4.0)
       concurrent-ruby (>= 1.0, < 2.0)
       json
       rouge (>= 3.30, < 5.0)