diff --git a/.travis.yml b/.travis.yml
index 3524135..2edc8ab 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,12 +10,9 @@ rvm:
   - "2.7"
   - "2.6"
   - "2.5"
-  - "2.4"
 gemfile:
   - gemfiles/Gemfile.rails-6.0.x
   - gemfiles/Gemfile.rails-5.2.x
-  - gemfiles/Gemfile.rails-5.1.x
-  - gemfiles/Gemfile.rails-5.0.x
 env:
   - TEST_TASK=spec
 matrix:
@@ -23,9 +20,6 @@ matrix:
     - rvm: ruby-head
   include:
     - { rvm: "2.6",   gemfile: "Gemfile", env: [TEST_TASK=rubocop] }
-  exclude:
-    # Rails > 5 not on MRI 2.4+
-    - { rvm: "2.4",   gemfile: "gemfiles/Gemfile.rails-6.0.x" }
 addons:
   apt:
     packages:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 03ffdb9..8d4e372 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,12 @@
 
 For the full commit log, [see here](https://github.com/influxdata/influxdb-rails/commits/master).
 
+## v1.0.1.beta2
+- Implement [`deliver.action_mailer`](https://guides.rubyonrails.org/active_support_instrumentation.html#deliver-action-mailer) subscriber
+- Add missing Active Job documentation
+- Drop support for Ruby 2.4
+- Drop support for Rails < 5.2
+
 ## v1.0.1.beta1, released 2020-08-21
 - Drop support for Ruby 2.3
 - Drop support for Rails 4.x
diff --git a/README.md b/README.md
index ddb96f0..fe25339 100644
--- a/README.md
+++ b/README.md
@@ -158,6 +158,27 @@ Reported tags:
 *Note*: Only the measurements with the hook `perform` report a duration in the value.
 The other hooks are counters and always report a value of `1`.
 
+### Action Mailer
+
+Reported ActiveSupport instrumentation hooks:
+
+- [deliver.action\_mailer](https://guides.rubyonrails.org/active_support_instrumentation.html#deliver-action-mailer)
+
+Reported values:
+
+```ruby
+  value: 1
+```
+
+Reported tags:
+
+```ruby
+  hook:               "deliver",
+  mailer:             "SomeMailerClassName"
+```
+
+*Note*: The hook is just a counter and always report a value of `1`.
+
 ## Configuration
 
 The only setting you actually need to configure is the name of the database
diff --git a/gemfiles/Gemfile.rails-5.0.x b/gemfiles/Gemfile.rails-5.0.x
deleted file mode 100644
index cca77eb..0000000
--- a/gemfiles/Gemfile.rails-5.0.x
+++ /dev/null
@@ -1,9 +0,0 @@
-source "https://rubygems.org"
-
-gem 'actionpack', '~> 5.0.0'
-gem 'activesupport', '~> 5.0.0'
-gem 'activemodel', '~> 5.0.0'
-gem 'sqlite3', '1.3.13'
-
-
-gemspec :path => '../'
diff --git a/gemfiles/Gemfile.rails-5.1.x b/gemfiles/Gemfile.rails-5.1.x
deleted file mode 100644
index 167e894..0000000
--- a/gemfiles/Gemfile.rails-5.1.x
+++ /dev/null
@@ -1,7 +0,0 @@
-source "https://rubygems.org"
-
-gem 'actionpack', '~> 5.1.0'
-gem 'activesupport', '~> 5.1.0'
-gem 'activemodel', '~> 5.1.0'
-
-gemspec :path => '../'
diff --git a/influxdb-rails.gemspec b/influxdb-rails.gemspec
index 3d54277..630850b 100644
--- a/influxdb-rails.gemspec
+++ b/influxdb-rails.gemspec
@@ -27,6 +27,7 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency "influxdb", "~> 0.6", ">= 0.6.4"
   spec.add_runtime_dependency "railties", ">= 5.0"
 
+  spec.add_development_dependency "actionmailer"
   spec.add_development_dependency "activejob"
   spec.add_development_dependency "activerecord"
   spec.add_development_dependency "bundler", ">= 1.0.0"
diff --git a/lib/influxdb-rails.rb b/lib/influxdb-rails.rb
index 8348781..54d06e9 100644
--- a/lib/influxdb-rails.rb
+++ b/lib/influxdb-rails.rb
@@ -8,6 +8,7 @@
 require "influxdb/rails/middleware/sql_subscriber"
 require "influxdb/rails/middleware/active_record_subscriber"
 require "influxdb/rails/middleware/active_job_subscriber"
+require "influxdb/rails/middleware/action_mailer_subscriber"
 require "influxdb/rails/sql/query"
 require "influxdb/rails/version"
 require "influxdb/rails/configuration"
diff --git a/lib/influxdb/rails/middleware/action_mailer_subscriber.rb b/lib/influxdb/rails/middleware/action_mailer_subscriber.rb
new file mode 100644
index 0000000..8100c2e
--- /dev/null
+++ b/lib/influxdb/rails/middleware/action_mailer_subscriber.rb
@@ -0,0 +1,22 @@
+require "influxdb/rails/middleware/subscriber"
+
+module InfluxDB
+  module Rails
+    module Middleware
+      class ActionMailerSubscriber < Subscriber # :nodoc:
+        private
+
+        def values
+          { value: 1 }
+        end
+
+        def tags
+          {
+            hook:   "deliver",
+            mailer: payload[:mailer],
+          }
+        end
+      end
+    end
+  end
+end
diff --git a/lib/influxdb/rails/railtie.rb b/lib/influxdb/rails/railtie.rb
index 17677a5..cd7ab6f 100644
--- a/lib/influxdb/rails/railtie.rb
+++ b/lib/influxdb/rails/railtie.rb
@@ -34,6 +34,7 @@ class Railtie < ::Rails::Railtie # :nodoc:
           "enqueue.active_job"                   => Middleware::ActiveJobSubscriber,
           "perform_start.active_job"             => Middleware::ActiveJobSubscriber,
           "perform.active_job"                   => Middleware::ActiveJobSubscriber,
+          "deliver.action_mailer"                => Middleware::ActionMailerSubscriber,
           "block_instrumentation.influxdb_rails" => Middleware::BlockInstrumentationSubscriber,
         }.each do |hook_name, subscriber|
           ActiveSupport::Notifications.subscribe(hook_name, subscriber)
diff --git a/spec/requests/action_mailer_deliver_metrics_spec.rb b/spec/requests/action_mailer_deliver_metrics_spec.rb
new file mode 100644
index 0000000..dec0cfa
--- /dev/null
+++ b/spec/requests/action_mailer_deliver_metrics_spec.rb
@@ -0,0 +1,49 @@
+require File.dirname(__FILE__) + "/../spec_helper"
+
+RSpec.describe "ActionMailer deliver metrics", type: :request do
+  let(:tags_middleware) do
+    lambda do |tags|
+      tags.merge(tags_middleware: :tags_middleware)
+    end
+  end
+  before do
+    allow_any_instance_of(InfluxDB::Rails::Configuration).to receive(:ignored_environments).and_return(%w[development])
+    allow_any_instance_of(ActionDispatch::Request).to receive(:request_id).and_return(:request_id)
+    allow_any_instance_of(InfluxDB::Rails::Configuration).to receive(:application_name).and_return(:app_name)
+    allow_any_instance_of(InfluxDB::Rails::Configuration).to receive(:tags_middleware).and_return(tags_middleware)
+  end
+
+  it "writes metric" do
+    get "/metrics"
+
+    expect_metric(
+      tags:   a_hash_including(
+        hook:            "deliver",
+        mailer:          "MetricMailer",
+        location:        "MetricsController#index",
+        additional_tag:  :value,
+        server:          Socket.gethostname,
+        app_name:        :app_name,
+        tags_middleware: :tags_middleware
+      ),
+      values: a_hash_including(
+        additional_value: :value,
+        request_id:       :request_id,
+        value:            1
+      )
+    )
+  end
+
+  it "does not write metric when hook is ignored" do
+    allow_any_instance_of(InfluxDB::Rails::Configuration).to receive(:ignored_hooks).and_return(["deliver.action_mailer"])
+
+    get "/metrics"
+
+    expect_no_metric(
+      tags: a_hash_including(
+        hook:   "deliver",
+        mailer: "MetricMailer"
+      )
+    )
+  end
+end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index e688f33..2dfd6b1 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -35,5 +35,6 @@
 
   config.include ActiveSupport::Testing::TimeHelpers
   config.include ActiveJob::TestHelper
+
   config.include InfluxDB::Rails::BrokenClient
 end
diff --git a/spec/support/rails5/app.rb b/spec/support/rails5/app.rb
index f061bf6..013b4ce 100644
--- a/spec/support/rails5/app.rb
+++ b/spec/support/rails5/app.rb
@@ -1,6 +1,7 @@
 require "action_controller/railtie"
 require "active_record/railtie"
 require "active_job"
+require "action_mailer"
 
 app = Class.new(Rails::Application)
 app.config.secret_key_base = "1234567890abcdef1234567890abcdef"
@@ -11,6 +12,7 @@
 app.config.root = __dir__
 Rails.backtrace_cleaner.remove_silencers!
 ActiveJob::Base.logger = Rails.logger
+ActionMailer::Base.delivery_method = :test
 app.initialize!
 
 app.routes.draw do
@@ -38,6 +40,17 @@ def perform
   end
 end
 
+class MetricMailer < ActionMailer::Base
+  default from: "from@example.com"
+  layout "mailer"
+
+  def welcome_mail
+    mail(to: "eisendieter@werder.de", subject: "Welcome to metrics!") do |format|
+      format.text { render plain: "Hello Dieter!" }
+    end
+  end
+end
+
 class Metric < ActiveRecord::Base; end
 class ApplicationController < ActionController::Base; end
 class MetricsController < ApplicationController
@@ -53,6 +66,7 @@ def index
       1 + 1
     end
     MetricJob.perform_later
+    MetricMailer.with(user: "eisendieter").welcome_mail.deliver_now
     Metric.create!(name: "name")
   end
 
diff --git a/spec/support/rails6/app.rb b/spec/support/rails6/app.rb
index f061bf6..013b4ce 100644
--- a/spec/support/rails6/app.rb
+++ b/spec/support/rails6/app.rb
@@ -1,6 +1,7 @@
 require "action_controller/railtie"
 require "active_record/railtie"
 require "active_job"
+require "action_mailer"
 
 app = Class.new(Rails::Application)
 app.config.secret_key_base = "1234567890abcdef1234567890abcdef"
@@ -11,6 +12,7 @@
 app.config.root = __dir__
 Rails.backtrace_cleaner.remove_silencers!
 ActiveJob::Base.logger = Rails.logger
+ActionMailer::Base.delivery_method = :test
 app.initialize!
 
 app.routes.draw do
@@ -38,6 +40,17 @@ def perform
   end
 end
 
+class MetricMailer < ActionMailer::Base
+  default from: "from@example.com"
+  layout "mailer"
+
+  def welcome_mail
+    mail(to: "eisendieter@werder.de", subject: "Welcome to metrics!") do |format|
+      format.text { render plain: "Hello Dieter!" }
+    end
+  end
+end
+
 class Metric < ActiveRecord::Base; end
 class ApplicationController < ActionController::Base; end
 class MetricsController < ApplicationController
@@ -53,6 +66,7 @@ def index
       1 + 1
     end
     MetricJob.perform_later
+    MetricMailer.with(user: "eisendieter").welcome_mail.deliver_now
     Metric.create!(name: "name")
   end
 
diff --git a/spec/support/views/layouts/mailer.txt.erb b/spec/support/views/layouts/mailer.txt.erb
new file mode 100644
index 0000000..37f0bdd
--- /dev/null
+++ b/spec/support/views/layouts/mailer.txt.erb
@@ -0,0 +1 @@
+<%= yield %>