Added optional reply to address

.env.sample

@@ -19,3 +19,4 @@ SMTP_PORT=
 SMTP_USERNAME=
 SMTP_PASSWORD=
 SMTP_SENDER_EMAIL=
+SMTP_REPLY_TO=

server/__snapshots__/mailer.test.js.snap

@@ -49,6 +49,7 @@ Object {
         </body>
       </html>
     ",
+  "replyTo": "hello@example.com",
   "subject": "Welcome to Outline",
   "text": "
 Welcome to Outline!

server/mailer.js

@@ -46,6 +46,7 @@ class Mailer {
       try {
         await transporter.sendMail({
           from: process.env.SMTP_SENDER_EMAIL,
+          replyTo: process.env.SMTP_REPLY_TO || process.env.SMTP_SENDER_EMAIL,
           to: data.to,
           subject: data.title,
           html: html,