diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e67a4b1..2d214c4 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -13,6 +13,8 @@ name: ci
   pull_request:
     branches:
       - main
+  schedule:
+    - cron: "30 4 * * *"
   create:
 
 jobs:
@@ -22,10 +24,16 @@ jobs:
       fail-fast: false
       matrix:
         ruby:
+          - "3.1"
           - "3.0"
-          - "2.7"
-          - "2.6"
     steps:
+      - uses: ravsamhq/notify-slack-action@v1
+        if: always()
+        with:
+          status: ${{ job.status }}
+          notify_when: "failure"
+        env:
+          SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}
       - uses: actions/checkout@v1
       - name: Install package dependencies
         run: "[ -e $APT_DEPS ] || sudo apt-get install -y --no-install-recommends $APT_DEPS"
diff --git a/README.md b/README.md
index d63a063..799c990 100644
--- a/README.md
+++ b/README.md
@@ -25,7 +25,7 @@ Mail for Ruby applications.
 
 ## Rubies
 
-__Hanami::Mailer__ supports Ruby (MRI) 2.6+
+__Hanami::Mailer__ supports Ruby (MRI) 3.0+
 
 ## Installation
 
diff --git a/hanami-mailer.gemspec b/hanami-mailer.gemspec
index 7776d9d..31acd8a 100644
--- a/hanami-mailer.gemspec
+++ b/hanami-mailer.gemspec
@@ -19,14 +19,20 @@ Gem::Specification.new do |spec|
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
-  spec.required_ruby_version = ">= 2.6.0"
+  spec.metadata["rubygems_mfa_required"] = "true"
+  spec.required_ruby_version = ">= 3.0"
 
   spec.add_dependency "hanami-utils", "~> 2.0.alpha"
   spec.add_dependency "tilt",         "~> 2.0", ">= 2.0.1"
-  spec.add_dependency "mail",         "~> 2.6"
+  spec.add_dependency "mail",         "~> 2.7"
+
+  # FIXME: remove when https://github.com/mikel/mail/pull/1439 gets merged AND a new version of `mail` gets released
+  spec.add_dependency "net-smtp", "~> 0.3"
+  spec.add_dependency "net-pop",  "~> 0.1"
+  spec.add_dependency "net-imap", "~> 0.2"
 
   spec.add_development_dependency "bundler", ">= 1.6", "< 3"
-  spec.add_development_dependency "rake",  "~> 13"
-  spec.add_development_dependency "rspec", "~> 3.7"
-  spec.add_development_dependency "rubocop", "0.91"
+  spec.add_development_dependency "rake",    "~> 13"
+  spec.add_development_dependency "rspec",   "~> 3.9"
+  spec.add_development_dependency "rubocop", "~> 1.0"
 end
diff --git a/spec/integration/hanami/mailer/delivery_spec.rb b/spec/integration/hanami/mailer/delivery_spec.rb
index 51c37cd..b22ffbd 100644
--- a/spec/integration/hanami/mailer/delivery_spec.rb
+++ b/spec/integration/hanami/mailer/delivery_spec.rb
@@ -70,8 +70,8 @@ RSpec.describe Hanami::Mailer do
 
         count.times do |i|
           threads << Thread.new do
-            user  = OpenStruct.new(name: "Luca #{i}", email: "luca-#{i}@domain.test")
-            event = OpenStruct.new(id: i, title: "Event ##{i}")
+            user  = double(name: "Luca #{i}", email: "luca-#{i}@domain.test")
+            event = double(id: i, title: "Event ##{i}")
 
             mails[i] = subject.deliver(user: user, event: event)
           end
diff --git a/spec/unit/hanami/mailer_spec.rb b/spec/unit/hanami/mailer_spec.rb
index 2c7a70a..3ec1c84 100644
--- a/spec/unit/hanami/mailer_spec.rb
+++ b/spec/unit/hanami/mailer_spec.rb
@@ -110,8 +110,8 @@ RSpec.describe Hanami::Mailer do
 
     context "locals" do
       let(:mailer) { EventMailer.new(configuration: configuration) }
-      let(:user)   { OpenStruct.new(name: "Luca", email: "luca@domain.test") }
-      let(:event)  { OpenStruct.new(id: 23, title: "Event #23") }
+      let(:user)   { double(name: "Luca", email: "luca@domain.test") }
+      let(:event)  { double(id: 23, title: "Event #23") }
 
       it "uses locals during the delivery process" do
         mail = mailer.deliver(user: user, event: event)