diff --git a/app/mailers/reminders/notification_mailer.rb b/app/mailers/reminders/notification_mailer.rb
index d0a2e0ed116..8fa0a8d9eff 100644
--- a/app/mailers/reminders/notification_mailer.rb
+++ b/app/mailers/reminders/notification_mailer.rb
@@ -75,7 +75,7 @@ class Reminders::NotificationMailer < ApplicationMailer
   end
 
   def work_package_subject_text_wrapper
-    "=" * ("# #{@work_package.id}#{@work_package.subject}".length + 4)
+    "=" * ("#{@work_package.formatted_id} #{@work_package.subject}".length + 4)
   end
 
   def text_email_wrapper
diff --git a/app/mailers/sharing_mailer.rb b/app/mailers/sharing_mailer.rb
index 917714e0cb0..af5f409d15c 100644
--- a/app/mailers/sharing_mailer.rb
+++ b/app/mailers/sharing_mailer.rb
@@ -49,7 +49,7 @@ class SharingMailer < ApplicationMailer
     send_localized_mail(@shared_with_user) do
       @role_rights = derive_role_rights(role)
       @allowed_work_package_actions = derive_allowed_work_package_actions(role)
-      I18n.t("mail.sharing.work_packages.subject", id: @work_package.id)
+      I18n.t("mail.sharing.work_packages.subject", id: @work_package.formatted_id)
     end
   end
 
diff --git a/app/mailers/work_package_mailer.rb b/app/mailers/work_package_mailer.rb
index 75a48ac27c5..f150e18834e 100644
--- a/app/mailers/work_package_mailer.rb
+++ b/app/mailers/work_package_mailer.rb
@@ -47,7 +47,7 @@ class WorkPackageMailer < ApplicationMailer
       send_localized_mail(recipient) do
         I18n.t(:"mail.mention.subject",
                user_name: author.name,
-               id: @work_package.id,
+               id: @work_package.formatted_id,
                subject: @work_package.subject)
       end
     end
@@ -73,7 +73,7 @@ class WorkPackageMailer < ApplicationMailer
 
   def subject_for_work_package(work_package)
     "#{work_package.project.name} - #{work_package.status.name} #{work_package.type.name} " +
-      "##{work_package.id}: #{work_package.subject}"
+      "#{work_package.formatted_id}: #{work_package.subject}"
   end
 
   def set_work_package_headers(work_package)
diff --git a/app/views/digest_mailer/work_packages.text.erb b/app/views/digest_mailer/work_packages.text.erb
index 5c2983d8d13..5d3a4c3c8d6 100644
--- a/app/views/digest_mailer/work_packages.text.erb
+++ b/app/views/digest_mailer/work_packages.text.erb
@@ -4,9 +4,9 @@
 
 <% @aggregated_notifications.first(DigestMailer::MAX_SHOWN_WORK_PACKAGES).each do |work_package, notifications_by_work_package| %>
 
-  <%= "=" * (("# " + work_package.id.to_s + work_package.subject).length + 4) %>
-  = #<%= work_package.id %> <%= work_package.subject %> =
-  <%= "=" * (("# " + work_package.id.to_s + work_package.subject).length + 4) %>
+  <%= "=" * ((work_package.formatted_id + " " + work_package.subject).length + 4) %>
+  = <%= work_package.formatted_id %> <%= work_package.subject %> =
+  <%= "=" * ((work_package.formatted_id + " " + work_package.subject).length + 4) %>
   
   <% unique_reasons = unique_reasons_of_notifications(notifications_by_work_package) %>
   <% unique_reasons.each_with_index do |reason, index| %><%= I18n.t(:"mail.work_packages.reason.#{reason || :unknown}", default: "-") %><%= ", " unless unique_reasons.size - 1 == index  %><% end %>: <% if notifications_by_work_package.first.date_alert? %>
diff --git a/app/views/mailer/_notification_row.html.erb b/app/views/mailer/_notification_row.html.erb
index 040820e5bdc..66ba12de800 100644
--- a/app/views/mailer/_notification_row.html.erb
+++ b/app/views/mailer/_notification_row.html.erb
@@ -27,7 +27,7 @@
                   </td>
                   <%= placeholder_cell("8px", vertical: true) %>
                   <td width="100%" style="color: #878787;">
-                    #<%= work_package.id %> - <%= work_package.project %>
+                    <%= work_package.formatted_id %> - <%= work_package.project %>
                     <%= " - " unless unique_reasons.length === 1 && unique_reasons.first.nil? %>
                     <% unique_reasons.each_with_index do |reason, index| %>
                       <%= I18n.t(
diff --git a/app/views/reminders/notification_mailer/reminder_notification.text.erb b/app/views/reminders/notification_mailer/reminder_notification.text.erb
index ce8e0d34e55..0230402630d 100644
--- a/app/views/reminders/notification_mailer/reminder_notification.text.erb
+++ b/app/views/reminders/notification_mailer/reminder_notification.text.erb
@@ -3,7 +3,7 @@
 <%= text_email_wrapper %>
 
 <%= work_package_subject_text_wrapper %>
-= #<%= @work_package.id %> <%= @work_package.subject %> =
+= <%= @work_package.formatted_id %> <%= @work_package.subject %> =
 <%= work_package_subject_text_wrapper %>
 
 <%= reminder_timestamp_text %>
diff --git a/app/views/work_package_mailer/mentioned.text.erb b/app/views/work_package_mailer/mentioned.text.erb
index c3ef81556b7..eee14ad5c8b 100644
--- a/app/views/work_package_mailer/mentioned.text.erb
+++ b/app/views/work_package_mailer/mentioned.text.erb
@@ -3,9 +3,9 @@
 <%= "-" * 100 %>
 
 
-<%= "=" * (("# " + @work_package.id.to_s + @work_package.subject).length + 4) %>
-= #<%= @work_package.id %> <%= @work_package.subject %> =
-<%= "=" * (("# " + @work_package.id.to_s + @work_package.subject).length + 4) %>
+<%= "=" * ((@work_package.formatted_id + " " + @work_package.subject).length + 4) %>
+= <%= @work_package.formatted_id %> <%= @work_package.subject %> =
+<%= "=" * ((@work_package.formatted_id + " " + @work_package.subject).length + 4) %>
 
 <%= I18n.t(:label_comment_added) %>:
 <%= strip_tags @journal.notes %>
diff --git a/app/views/work_package_mailer/watcher_changed.html.erb b/app/views/work_package_mailer/watcher_changed.html.erb
index 9be970cecbc..6089c2dd7b9 100644
--- a/app/views/work_package_mailer/watcher_changed.html.erb
+++ b/app/views/work_package_mailer/watcher_changed.html.erb
@@ -26,7 +26,7 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 See COPYRIGHT and LICENSE files for more details.
 
 ++#%>
-<%= t("text_work_package_watcher_#{@action}", id: "##{@work_package.id}", watcher_changer: @watcher_changer) %>
+<%= t("text_work_package_watcher_#{@action}", id: @work_package.formatted_id, watcher_changer: @watcher_changer) %>
 <hr>
 <%= render partial: "work_package_details", locals: { work_package: @work_package } %>
 <p>
diff --git a/app/views/work_package_mailer/watcher_changed.text.erb b/app/views/work_package_mailer/watcher_changed.text.erb
index 289e99f6a79..7de3fc8bccc 100644
--- a/app/views/work_package_mailer/watcher_changed.text.erb
+++ b/app/views/work_package_mailer/watcher_changed.text.erb
@@ -27,7 +27,7 @@ See COPYRIGHT and LICENSE files for more details.
 
 ++#%>
 
-<%= t("text_work_package_watcher_#{@action}", id: "##{@work_package.id}", watcher_changer: @watcher_changer) %>
+<%= t("text_work_package_watcher_#{@action}", id: @work_package.formatted_id, watcher_changer: @watcher_changer) %>
 
 ----------------------------------------
 <%= render partial: "work_package_details", locals: { work_package: @work_package } %>
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 0319501ef85..db283a32309 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -4615,7 +4615,7 @@ en:
       you_have: "You have"
     logo_alt_text: "Logo"
     mention:
-      subject: "%{user_name} mentioned you in #%{id} - %{subject}"
+      subject: "%{user_name} mentioned you in %{id} - %{subject}"
     notification:
       center: "To notification center"
       see_in_center: "See comment in notification center"
@@ -4653,7 +4653,7 @@ en:
         allowed_actions_html: "You have the following permissions on this work package: %{allowed_actions}. This can change depending on your project role and permissions."
         create_account: "To access this work package, you will need to create and activate an account on %{instance}."
         open_work_package: "Open work package"
-        subject: "Work package #%{id} was shared with you"
+        subject: "Work package %{id} was shared with you"
         enterprise_text: "Share work packages with users who are not members of the project."
         summary:
           user: "%{user} shared a work package with you with %{role_rights} rights"
diff --git a/spec/factories/project_factory.rb b/spec/factories/project_factory.rb
index 22566def703..0ada4e740bd 100644
--- a/spec/factories/project_factory.rb
+++ b/spec/factories/project_factory.rb
@@ -49,7 +49,7 @@ FactoryBot.define do
     workspace_type { "project" }
 
     sequence(:name) { |n| "My Project No. #{n}" }
-    sequence(:identifier) { |n| "myproject_no_#{n}" }
+    sequence(:identifier) { |n| Setting::WorkPackageIdentifier.semantic_mode_active? ? "MPN#{n}" : "myproject_no_#{n}" }
 
     # Use this trait for specs that exercise semantic-mode behaviour.
     # Produces a deterministic uppercase identifier that satisfies
diff --git a/spec/features/notifications/settings/immediate_reminder_spec.rb b/spec/features/notifications/settings/immediate_reminder_spec.rb
index e483ed632c4..dddb8678073 100644
--- a/spec/features/notifications/settings/immediate_reminder_spec.rb
+++ b/spec/features/notifications/settings/immediate_reminder_spec.rb
@@ -104,7 +104,7 @@ RSpec.describe "Immediate reminder settings", :js do
       expect(ActionMailer::Base.deliveries.first.subject)
         .to eql I18n.t(:"mail.mention.subject",
                        user_name: current_user.name,
-                       id: work_package.id,
+                       id: "##{work_package.id}",
                        subject: work_package.subject)
     end
   end
diff --git a/spec/mailers/digest_mailer_spec.rb b/spec/mailers/digest_mailer_spec.rb
index 1723da82586..3ab8b3bebaa 100644
--- a/spec/mailers/digest_mailer_spec.rb
+++ b/spec/mailers/digest_mailer_spec.rb
@@ -89,13 +89,26 @@ RSpec.describe DigestMailer do
       expected_notification_subject = "#{work_package.type.name.upcase} #{work_package.subject}"
       expect(mail_body).to have_text(expected_notification_subject, normalize_ws: true)
 
-      expected_notification_header = "#{work_package.status.name} ##{work_package.id} - #{work_package.project}"
-      expect(mail_body).to have_text(expected_notification_header, normalize_ws: true)
-
       expected_text = "#{journal.initial? ? 'Created' : 'Updated'} at #{time_stamp} by #{recipient.name}"
       expect(mail_body).to have_text(expected_text, normalize_ws: true)
     end
 
+    context "with classic mode", with_settings: { work_packages_identifier: "classic" } do
+      it "shows the # prefixed numeric id in the notification header" do
+        expected = "#{work_package.status.name} ##{work_package.id} - #{work_package.project}"
+        expect(mail_body).to have_text(expected, normalize_ws: true)
+      end
+    end
+
+    context "with semantic mode",
+            with_flag: { semantic_work_package_ids: true },
+            with_settings: { work_packages_identifier: "semantic" } do
+      it "shows the semantic identifier without # prefix in the notification header" do
+        expected = "#{work_package.status.name} #{work_package.identifier} - #{work_package.project}"
+        expect(mail_body).to have_text(expected, normalize_ws: true)
+      end
+    end
+
     it "includes a reference to the notification center if there are more than the maximum number of shown work packages" do
       stub_const("DigestMailer::MAX_SHOWN_WORK_PACKAGES", 0)
 
diff --git a/spec/mailers/reminders/notification_mailer_spec.rb b/spec/mailers/reminders/notification_mailer_spec.rb
index ad0a5de75da..294aa544dda 100644
--- a/spec/mailers/reminders/notification_mailer_spec.rb
+++ b/spec/mailers/reminders/notification_mailer_spec.rb
@@ -81,5 +81,24 @@ RSpec.describe Reminders::NotificationMailer do
     it "mail body includes the reminder note" do
       expect(mail_body).to include("Note: “This is an important reminder”")
     end
+
+    context "with classic mode", with_settings: { work_packages_identifier: "classic" } do
+      it "shows the # prefixed numeric id in the mail body" do
+        expect(mail_body).to include("##{work_package.id}")
+      end
+    end
+
+    context "with semantic mode",
+            with_flag: { semantic_work_package_ids: true },
+            with_settings: { work_packages_identifier: "semantic" } do
+      let(:work_package) do
+        build_stubbed(:work_package, identifier: "PROJ-42", sequence_number: 42)
+      end
+
+      it "shows the semantic identifier without # prefix in the mail body" do
+        expect(mail_body).to include("PROJ-42")
+        expect(mail_body).not_to include("#PROJ-42")
+      end
+    end
   end
 end
diff --git a/spec/mailers/sharing_mailer_spec.rb b/spec/mailers/sharing_mailer_spec.rb
index 90be2256fd3..c0337fe757d 100644
--- a/spec/mailers/sharing_mailer_spec.rb
+++ b/spec/mailers/sharing_mailer_spec.rb
@@ -59,10 +59,29 @@ RSpec.describe SharingMailer do
         .to contain_exactly(shared_with_user.mail)
     end
 
-    it "sets the appropriate subject" do
-      expect(mail.subject)
-        .to eq(I18n.t("mail.sharing.work_packages.subject",
-                      id: work_package.id))
+    context "with classic mode", with_settings: { work_packages_identifier: "classic" } do
+      it "sets the subject with # prefixed numeric id" do
+        expect(mail.subject)
+          .to eq(I18n.t("mail.sharing.work_packages.subject", id: "##{work_package.id}"))
+      end
+    end
+
+    context "with semantic mode",
+            with_flag: { semantic_work_package_ids: true },
+            with_settings: { work_packages_identifier: "semantic" } do
+      let(:work_package) do
+        build_stubbed(:work_package,
+                      type: build_stubbed(:type_standard),
+                      author: build_stubbed(:user),
+                      project:,
+                      identifier: "PROJ-42",
+                      sequence_number: 42)
+      end
+
+      it "sets the subject with the semantic identifier without # prefix" do
+        expect(mail.subject)
+          .to eq(I18n.t("mail.sharing.work_packages.subject", id: "PROJ-42"))
+      end
     end
 
     it "has a project header" do
diff --git a/spec/mailers/work_package_mailer_spec.rb b/spec/mailers/work_package_mailer_spec.rb
index 4446cb898c2..a2a73113885 100644
--- a/spec/mailers/work_package_mailer_spec.rb
+++ b/spec/mailers/work_package_mailer_spec.rb
@@ -55,12 +55,35 @@ RSpec.describe WorkPackageMailer do
   describe "#mentioned" do
     subject(:mail) { described_class.mentioned(recipient, journal) }
 
-    it "has a subject" do
-      expect(mail.subject)
-        .to eql I18n.t(:"mail.mention.subject",
-                       user_name: author.name,
-                       id: work_package.id,
-                       subject: work_package.subject)
+    context "with classic mode", with_settings: { work_packages_identifier: "classic" } do
+      it "has a subject with # prefixed numeric id" do
+        expect(mail.subject)
+          .to eql I18n.t(:"mail.mention.subject",
+                         user_name: author.name,
+                         id: "##{work_package.id}",
+                         subject: work_package.subject)
+      end
+    end
+
+    context "with semantic mode",
+            with_flag: { semantic_work_package_ids: true },
+            with_settings: { work_packages_identifier: "semantic" } do
+      let(:work_package) do
+        build_stubbed(:work_package,
+                      type: build_stubbed(:type_standard),
+                      project:,
+                      assigned_to: assignee,
+                      identifier: "PROJ-42",
+                      sequence_number: 42)
+      end
+
+      it "has a subject with semantic identifier and no # prefix" do
+        expect(mail.subject)
+          .to eql I18n.t(:"mail.mention.subject",
+                         user_name: author.name,
+                         id: "PROJ-42",
+                         subject: work_package.subject)
+      end
     end
 
     it "is sent to the recipient" do
@@ -122,6 +145,30 @@ RSpec.describe WorkPackageMailer do
           .to include(work_package.subject)
       end
 
+      context "with classic mode", with_settings: { work_packages_identifier: "classic" } do
+        it "includes the # prefixed numeric id in the subject" do
+          expect(mail.subject).to include("##{work_package.id}")
+        end
+      end
+
+      context "with semantic mode",
+              with_flag: { semantic_work_package_ids: true },
+              with_settings: { work_packages_identifier: "semantic" } do
+        let(:work_package) do
+          build_stubbed(:work_package,
+                        type: build_stubbed(:type_standard),
+                        project:,
+                        assigned_to: assignee,
+                        identifier: "PROJ-42",
+                        sequence_number: 42)
+        end
+
+        it "includes the semantic identifier without # prefix in the subject" do
+          expect(mail.subject).to include("PROJ-42")
+          expect(mail.subject).not_to match(/#PROJ-42/)
+        end
+      end
+
       it "has a references header" do
         expect(mail.references)
           .to eql "op.work_package-#{work_package.id}@example.net"
@@ -136,6 +183,30 @@ RSpec.describe WorkPackageMailer do
           .to include(work_package.subject)
       end
 
+      context "with classic mode", with_settings: { work_packages_identifier: "classic" } do
+        it "includes the # prefixed numeric id in the subject" do
+          expect(mail.subject).to include("##{work_package.id}")
+        end
+      end
+
+      context "with semantic mode",
+              with_flag: { semantic_work_package_ids: true },
+              with_settings: { work_packages_identifier: "semantic" } do
+        let(:work_package) do
+          build_stubbed(:work_package,
+                        type: build_stubbed(:type_standard),
+                        project:,
+                        assigned_to: assignee,
+                        identifier: "PROJ-42",
+                        sequence_number: 42)
+        end
+
+        it "includes the semantic identifier without # prefix in the subject" do
+          expect(mail.subject).to include("PROJ-42")
+          expect(mail.subject).not_to match(/#PROJ-42/)
+        end
+      end
+
       it "has a references header" do
         expect(mail.references)
           .to eql "op.work_package-#{work_package.id}@example.net"