diff --git a/.changeset/emails-integration-option.md b/.changeset/emails-integration-option.md
new file mode 100644
index 00000000..ef342989
--- /dev/null
+++ b/.changeset/emails-integration-option.md
@@ -0,0 +1,5 @@
+---
+"posthog-ruby": minor
+---
+
+`client.emails.send` takes `integration:`, for a library that sends email on the app's behalf, such as a mailer adapter. Pass its name and version, such as `integration: 'action-mailer/3.14.0'`, and the send carries it as the `X-PostHog-Integration` header, so PostHog can tell which integration sent the email. A value outside `name/version` raises `ArgumentError` before any request. The User-Agent doesn't change.
diff --git a/lib/posthog/emails.rb b/lib/posthog/emails.rb
index 59e2e13c..e9dc9ace 100644
--- a/lib/posthog/emails.rb
+++ b/lib/posthog/emails.rb
@@ -84,6 +84,7 @@ class Emails
     IDEMPOTENCY_KEY_PATTERN = /\A[\x21-\x7e]{1,255}\z/
     TEMPLATE_KEY_PATTERN = /\A[a-zA-Z0-9_-]{1,400}\z/
     HEADER_TOKEN_PATTERN = /\A[\x21-\x7e]+\z/
+    INTEGRATION_PATTERN = %r{\A[a-z0-9-]{1,40}/[0-9A-Za-z.+-]{1,40}\z}
     SEND_KEY_NEEDED = 'Sending email needs secret_key, a project secret API key with the hog_flow:write scope.'
     READ_KEY_NEEDED = 'Reading an email status needs secret_key, a project secret API key with the hog_flow:read ' \
                       'or hog_flow:write scope.'
@@ -100,9 +101,9 @@ class Emails
     ].freeze
     private_constant :SEND_PATH, :STATUS_PATH, :US_APP_HOST, :EU_APP_HOST, :CLOUD_APP_HOSTS,
                      :MAX_RETRY_DELAY_SECONDS, :UUID_PATTERN, :IDEMPOTENCY_KEY_PATTERN, :TEMPLATE_KEY_PATTERN,
-                     :HEADER_TOKEN_PATTERN, :SEND_KEY_NEEDED, :READ_KEY_NEEDED, :TEMPLATE_FORM_WHY,
-                     :CODE_FORM_WHY, :TEMPLATE_FIELDS, :CODE_FIELDS, :ATTACHMENT_FIELDS, :BASE64_PATTERN,
-                     :NETWORK_ERRORS
+                     :HEADER_TOKEN_PATTERN, :INTEGRATION_PATTERN, :SEND_KEY_NEEDED, :READ_KEY_NEEDED,
+                     :TEMPLATE_FORM_WHY, :CODE_FORM_WHY, :TEMPLATE_FIELDS, :CODE_FIELDS, :ATTACHMENT_FIELDS,
+                     :BASE64_PATTERN, :NETWORK_ERRORS
 
     # @api private
     def initialize(secret_key:, host:, app_host:, timeout:, max_retries:, migrated_from:, user_agent:, disabled:,
@@ -162,6 +163,9 @@ def initialize(secret_key:, host:, app_host:, timeout:, max_retries:, migrated_f
     #   retries the send.
     # @param max_retries [Integer] Overrides the client's `email_request_max_retries` for this send. +0+ turns
     #   retries off.
+    # @param integration [String] For a library that sends on the app's behalf, such as a mailer adapter: its name
+    #   and version, such as `action-mailer/3.14.0`, sent as the `X-PostHog-Integration` header. The name is lowercase
+    #   letters, digits and hyphens; the version is letters, digits, dots, plus signs and hyphens; each at most 40.
     # @return [SendResult] The queued email.
     # @raise [EmailError] PostHog did not queue the email, or the client could not tell whether it did.
     #   {EmailError#code} is `invalid_attachment`, before any request, for a file the client can't send.
@@ -169,17 +173,19 @@ def initialize(secret_key:, host:, app_host:, timeout:, max_retries:, migrated_f
     def send(
       to:, template_id: nil, template_data: nil, template_key: nil, subject: nil, html: nil, text: nil, from: nil,
       reply_to: nil, cc: nil, bcc: nil, attachments: nil, capture_engagement: nil, distinct_id: nil,
-      idempotency_key: nil, max_retries: nil
+      idempotency_key: nil, max_retries: nil, integration: nil
     )
       fields = {
         template_data: template_data, subject: subject, html: html, text: text, from: from, reply_to: reply_to,
         cc: cc, bcc: bcc, attachments: attachments, capture_engagement: capture_engagement
       }.compact
       check_recipient(to, distinct_id, idempotency_key, max_retries)
+      check_integration(integration)
       payload = content(template_id, template_key, fields).merge(to: to)
       payload[:distinct_id] = distinct_id unless distinct_id.nil?
       body = JSON.generate(payload)
       headers = { 'Content-Type' => 'application/json', 'Idempotency-Key' => idempotency_key || SecureRandom.uuid }
+      headers['X-PostHog-Integration'] = integration if integration
 
       result = call('no email was sent', SEND_KEY_NEEDED) do |secret_key|
         uri = URI(@app_host + SEND_PATH)
@@ -280,6 +286,12 @@ def check_recipient(to, distinct_id, idempotency_key, max_retries)
       raise ArgumentError, 'max_retries must be a nonnegative integer.'
     end
 
+    def check_integration(integration)
+      return if integration.nil? || (integration.is_a?(String) && INTEGRATION_PATTERN.match?(integration))
+
+      raise ArgumentError, "integration must be a name and version, such as 'action-mailer/3.14.0'."
+    end
+
     def nonempty_string?(value)
       value.is_a?(String) && !value.strip.empty?
     end
diff --git a/public_api_snapshot.txt b/public_api_snapshot.txt
index 4417a854..50d9d7e3 100644
--- a/public_api_snapshot.txt
+++ b/public_api_snapshot.txt
@@ -74,7 +74,7 @@ instance_method PostHog::EmailError#status_code()
 instance_method PostHog::EmailError#why()
 class PostHog::Emails
 instance_method PostHog::Emails#get(message_id)
-instance_method PostHog::Emails#send(to:, template_id: ..., template_data: ..., template_key: ..., subject: ..., html: ..., text: ..., from: ..., reply_to: ..., cc: ..., bcc: ..., attachments: ..., capture_engagement: ..., distinct_id: ..., idempotency_key: ..., max_retries: ...)
+instance_method PostHog::Emails#send(to:, template_id: ..., template_data: ..., template_key: ..., subject: ..., html: ..., text: ..., from: ..., reply_to: ..., cc: ..., bcc: ..., attachments: ..., capture_engagement: ..., distinct_id: ..., idempotency_key: ..., max_retries: ..., integration: ...)
 class_method PostHog::Emails.logger()
 class PostHog::Emails::SendResult < Struct
 instance_method PostHog::Emails::SendResult#id()
diff --git a/spec/posthog/emails_spec.rb b/spec/posthog/emails_spec.rb
index e455a603..010c455b 100644
--- a/spec/posthog/emails_spec.rb
+++ b/spec/posthog/emails_spec.rb
@@ -330,6 +330,34 @@ def refused_attachment(**overrides)
       end
     end
 
+    describe 'integration marker' do
+      before { stub_request(:post, send_url).to_return(refuse(503, 'delivery_unavailable'), accept) }
+
+      it 'names the integration that made the send on every attempt, and keeps the User-Agent' do
+        send_code(integration: 'action-mailer/3.14.0')
+
+        expect(a_request(:post, send_url).with(
+                 headers: { 'X-PostHog-Integration' => 'action-mailer/3.14.0',
+                            'User-Agent' => "posthog-ruby/#{PostHog::VERSION}" }
+               )).to have_been_made.twice
+      end
+
+      it 'sends no marker without the option' do
+        send_template
+
+        expect(a_request(:post, send_url).with { |request| request.headers.key?('X-Posthog-Integration') })
+          .not_to have_been_made
+      end
+
+      it 'refuses a marker outside name/version before any request' do
+        ['action-mailer', 'Action-Mailer/1.0', 'action mailer/1.0', 'action-mailer/1.0 beta', "#{'a' * 41}/1.0", 7]
+          .each do |integration|
+            expect { send_code(integration: integration) }.to raise_error(ArgumentError, /integration must be/)
+          end
+        expect(a_request(:any, %r{/messaging/})).not_to have_been_made
+      end
+    end
+
     describe 'client setup' do
       it 'maps an EU ingestion host to the EU app host' do
         options[:host] = 'https://eu.i.posthog.com/'