diff --git a/calendar/experiments/calendar/parent/ext-calendar-provider.js b/calendar/experiments/calendar/parent/ext-calendar-provider.js
index b00dbec..a5f6e3d 100644
--- a/calendar/experiments/calendar/parent/ext-calendar-provider.js
+++ b/calendar/experiments/calendar/parent/ext-calendar-provider.js
@@ -5,11 +5,12 @@
 var { ExtensionCommon } = ChromeUtils.import("resource://gre/modules/ExtensionCommon.jsm");
 var { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");
 var { cal } = ChromeUtils.import("resource:///modules/calendar/calUtils.jsm");
+var { jsmime } = ChromeUtils.import("resource:///modules/jsmime.jsm");
 
 var { ExtensionAPI, EventManager } = ExtensionCommon;
 
 class ExtCalendarProvider extends cal.provider.BaseClass {
-  QueryInterface = ChromeUtils.generateQI(["calICalendar", "calIChangeLog", "calISchedulingSupport"]);
+  QueryInterface = ChromeUtils.generateQI(["calICalendar", "calIChangeLog", "calISchedulingSupport", "calIItipTransport"]);
 
   static register(extension) {
     let calmgr = cal.getCalendarManager();
@@ -117,11 +118,38 @@ class ExtCalendarProvider extends cal.provider.BaseClass {
           : ["unsubscribe"];
       case "requiresNetwork":
         return !(this.capabilities.requires_network === false);
+      case "itip.transport":
+        if (this.extension.emitter.has("calendar.provider.onBeforeSend")) {
+          return this;
+        }
+        break;
     }
 
     return super.getProperty(name);
   }
 
+  scheme = "mailto";
+
+  async sendItems(aRecipients, aItipItem) {
+    let transport = super.getProperty("itip.transport").wrappedJSObject;
+    let { subject, body } = transport._prepareItems(aItipItem);
+    let recipients = aRecipients.map(attendee => ({ name: attendee.commonName, email: attendee.id.replace(/^mailto:/, "")}));
+    let responses = await this.extension.emit("calendar.provider.onBeforeSend", this, aItipItem, { recipients, subject, body });
+    for (let response of responses) {
+      if (response === true) {
+        return;
+      }
+      if (Array.isArray(response?.recipients)) {
+        for (let recipient of response.recipients) {
+          aRecipients = aRecipients.filter(attendee => attendee.id.replace(/^mailto:/, "") != recipient.email);
+        }
+      }
+    }
+    if (aRecipients.length) {
+      await transport.sendItems(aRecipients, aItipItem);
+    }
+  }
+
   addItem(aItem, aListener) {
     return this.adoptItem(aItem.clone(), aListener);
   }
@@ -453,6 +481,54 @@ this.calendar_provider = class extends ExtensionAPI {
             },
           }).api(),
 
+          onBeforeSend: new EventManager({
+            context,
+            name: "calendar.provider.onBeforeSend",
+            register: (fire, options) => {
+              let listener = (event, calendar, itipItem, content) => {
+                content.responseMethod = itipItem.responseMethod;
+                if (["ics", "mime"].includes(options?.returnFormat)) {
+                  let serializer = Cc["@mozilla.org/calendar/ics-serializer;1"].createInstance(Ci.calIIcsSerializer);
+                  serializer.addItems(itipItem.getItemList());
+                  let methodProp = cal.getIcsService().createIcalProperty("METHOD");
+                  methodProp.value = content.responseMethod;
+                  serializer.addProperty(methodProp);
+                  let icsText = serializer.serializeToString();
+                  if (options.returnFormat == "mime") {
+                    let boundary = Array.from({ length: 24 }, () => Math.floor(Math.random() * 16).toString(16)).join("");
+                    let headers = new Map();
+                    headers.set("Date", [new Date()]);
+                    headers.set("Subject", [content.subject]);
+                    headers.set("To", content.recipients);
+                    headers.set("Content-Type", ["multipart/mixed; boundary=\"" + boundary + "\""]);
+                    let mimeContent = jsmime.headeremitter.emitStructuredHeaders(headers, { hardMargin: 800 }) + "\r\n";
+                    mimeContent += "--" + boundary + "\r\n";
+                    mimeContent += "Content-Type: text/plain; charset=UTF-8\r\n\r\n";
+                    mimeContent += content.body + "\r\n";
+                    mimeContent += "--" + boundary + "\r\n";
+                    mimeContent += "Content-Type: text/calendar; method=" + content.responseMethod + "; charset=UTF-8\r\nContent-Transfer-Encoding: 8BIT\r\n\r\n";
+                    mimeContent += icsText;
+                    mimeContent += "--" + boundary + "\r\n";
+                    mimeContent += "Content-Type: application/ics; name=\"invite.ics\"\r\nContent-Disposition: attachment; filename=\"invite.ics\"\r\nContent-Transfer-Encoding: 8BIT\r\n\r\n";
+                    mimeContent += icsText;
+                    mimeContent += "--" + boundary + "--\r\n";
+                    content.mimeContent = mimeContent;
+                  } else {
+                    content.icsText = icsText;
+                  }
+                } else {
+                  content.items = itipItem.getItemList().map(item => convertItem(item, options, context.extension));
+                }
+                return fire.async(convertCalendar(context.extension, calendar), content);
+              };
+
+              context.extension.on("calendar.provider.onBeforeSend", listener);
+              return () => {
+                context.extension.off("calendar.provider.onBeforeSend", listener);
+              };
+            },
+          }).api(),
+
           onFreeBusy: new EventManager({
             context,
             name: "calendar.provider.onFreeBusy",
diff --git a/calendar/experiments/calendar/schema/calendar-provider.json b/calendar/experiments/calendar/schema/calendar-provider.json
index 7fc8ee4..20ac293 100644
--- a/calendar/experiments/calendar/schema/calendar-provider.json
+++ b/calendar/experiments/calendar/schema/calendar-provider.json
@@ -87,6 +87,48 @@
           { "name": "calendar", "$ref": "calendar.calendars.Calendar" }
         ]
       },
+      {
+        "name": "onBeforeSend",
+        "type": "function",
+        "parameters": [
+          { "name": "calendar", "$ref": "calendar.calendars.Calendar" },
+          {
+            "name": "content",
+            "type": "object",
+            "properties": {
+              "responseMethod": { "type": "string" },
+              "recipients": {
+                "type": "array",
+                "items": {
+                  "type": "object",
+                  "properties": {
+                    "name": { "type": "string" },
+                    "email": { "type": "string" }
+                  }
+                }
+              },
+              "subject": { "type": "string" },
+              "body": { "type": "string" },
+              "items": { "type": "array", "items": { "$ref": "calendar.items.CalendarItem" }, "optional": true },
+              "icsText": { "type": "string", "optional": true },
+              "mimeContent": { "type": "string", "optional": true }
+            }
+          }
+        ],
+        "extraParameters": [
+          {
+            "type": "object",
+            "optional": true,
+            "properties": {
+              "returnFormat": {
+                "optional": true,
+                "type": "string",
+                "enum": ["ical", "jcal", "ics", "mime"]
+              }
+            }
+          }
+        ]
+      },
       {
         "name": "onFreeBusy",
         "type": "function",