diff --git a/README.md b/README.md
index 99bd03ad..4e473eeb 100644
--- a/README.md
+++ b/README.md
@@ -42,6 +42,8 @@ Some features:
     bcc: r2d2@example.com,hansolo@example.com
     # Optional recipient of the email response:
     reply_to: luke@example.com
+    # Optional Message ID this message is replying to:
+    in_reply_to: <random-luke@example.com>
     # Optional unsigned/invalid certificates allowance:
     ignore_cert: true
     # Optional converting Markdown to HTML (set content_type to text/html too):
diff --git a/action.yml b/action.yml
index bd9a09c8..33593b32 100644
--- a/action.yml
+++ b/action.yml
@@ -44,6 +44,9 @@ inputs:
   reply_to:
     description: An email address that will appear on the Reply-To field
     required: false
+  in_reply_to:
+    description: The Message-ID this message is replying to
+    required: false
   ignore_cert:
     description: Allow unsigned/invalid certificates
     required: false
diff --git a/main.js b/main.js
index 5707da5d..14c932c3 100644
--- a/main.js
+++ b/main.js
@@ -51,6 +51,7 @@ async function main() {
         const cc = core.getInput("cc", { required: false })
         const bcc = core.getInput("bcc", { required: false })
         const replyTo = core.getInput("reply_to", { required: false })
+        const inReplyTo = core.getInput("in_reply_to", { required: false })
         const attachments = core.getInput("attachments", { required: false })
         const convertMarkdown = core.getInput("convert_markdown", { required: false })
         const ignoreCert = core.getInput("ignore_cert", { required: false })
@@ -80,6 +81,8 @@ async function main() {
             cc: cc ? cc : undefined,
             bcc: bcc ? bcc : undefined,
             replyTo: replyTo ? replyTo : undefined,
+            inReplyTo: inReplyTo ? inReplyTo : undefined,
+            references: inReplyTo ? inReplyTo : undefined,
             text: body ? getBody(body, false) : undefined,
             html: htmlBody ? getBody(htmlBody, convertMarkdown) : undefined,
             priority: priority ? priority : undefined,