If you want sendgrid to make a REST api call to your server upon receiving an incoming email, follow these steps:

When sending an email, designate the reply-to email as something like @chat-reply.myserver.com

Then, create an endpoint in your server code to handle these requests. Your code will have to search for the conversation based on the incoming address and then save a record in the chat.

Below is some sample code:

import { Meteor } from 'meteor/meteor'
formidable = require('formidable');     // Formidable for form/file parsing
import { Profiles } from '../imports/api/collections';
import { inboundReply } from '../imports/api/inbound/methods.js';

const debug = require('debug')('myapp:inbound')

// This needs to run on both the server and client, hence not in the routing.js file
//   which only runs on the client.
//   Handles inbound emails for loop replies
//
// This is a RESTAPI end point called by sendgrid,
//   any email to <a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="cbb3b3b3b38ba8a3aabfe6b9aebba7b2e5a6b2b8aeb9bdaeb9e5a8a4a6e5aabe">[email protected]</a> will land here. Our task
//   is to parse it, determine the related loop, and store it as a message in the database
//
Router.route('/inbound', function () {

    // Use formidable because SendGrid inbound data is encoded as multipart/form-data
    const form = new formidable.IncomingForm();
    // Meteor bind environment to get callback
    debug(this.request.body)
    let r = this.response
    form.parse(this.request, Meteor.bindEnvironment(function (error, fields, files) {
      if (error)
        console.error(error);
      let errs = []

      // Get the 'to' field
      const toField = _.find(fields, function(value, key) { if (key === 'to') { return value; }});

      // Get the 'from' field
      const fromField = _.find(fields, function(value, key) { if (key === 'from') { return value; }});

      // Get the html content of the email
      const content = _.find(fields, function(value, key) { if (key === 'text') { return value; }});

      let cleanContent;
      if (content){
        // Logger.trace({content: content});
        // Regex removes html tags
        // cleanContent  = content.replace(/<br>/ig, "\n");
        // const regex = /(<([^>]+)>)/ig
        // cleanContent  = cleanContent.replace(regex, "");
        // Logger.trace({cleanContent: cleanContent});
        let lines = content.split(/\n/);
        debug("Incoming body",lines);