diff --git a/docs/en/04_Changelogs/4.9.0.md b/docs/en/04_Changelogs/4.9.0.md
index 087037f65..5914a4a23 100644
--- a/docs/en/04_Changelogs/4.9.0.md
+++ b/docs/en/04_Changelogs/4.9.0.md
@@ -5,6 +5,7 @@
 - [Features and enhancements](#features-and-enhancements)
     - [Image lazy loading](#image-lazy-loading)
     - [Manage your CMS sessions across devices](#session-manager)
+    - [Default mail transport upgraded to sendmail](#sendmail)
     - [Other new features](#other-features)
 - [Bugfixes](#bugfixes)
 
@@ -68,6 +69,16 @@ If your site has the [symbiote/silverstripe-queuedjobs](https://github.com/symbi
 
 CMS users can review the [Session Manager user help](https://userhelp.silverstripe.org/en/4/managing_your_website/session_manager/) for more information on managing their sessions.
 
+### Default mail transport upgraded to sendmail {#sendmail}
+
+Silverstripe CMS provides an API over the top of the [SwiftMailer](http://swiftmailer.org/) PHP library which comes with an extensive list of "transports" for sending mail via different services. 
+
+Prior to 4.9.0, Silverstripe CMS 4 defaulted to using the built-in PHP `mail()` command via a deprecated class `Swift_MailTransport`. However, using this layer is less secure and is strongly discouraged.
+
+Installations of Silverstripe CMS setup using silverstripe/installer 4.9.0 or greater default to using the more secure class `Swift_SendmailTransport` which uses a `sendmail` binary.
+
+It's highly recommended that existing Silverstripe CMS installation still using `Swift_MailTransport` upgrade to using `Swift_SendmailTransport` or another available transport, such as `Swift_SmtpTransport`. Details on how to use these classes are available in the [email section](https://docs.silverstripe.org/en/4/developer_guides/email/) of the developer docs.
+
 ### Other new features
 
 * [Dot notation support in form fields](https://github.com/silverstripe/silverstripe-framework/pull/9192): Save directly into nested has_one relationships (see [docs](/developer_guides/forms/how_tos/handle_nested_data)).