[Email Security] Add TTL note (cloudflare#22465)

* [Email Security] Add TTL note

* Update src/content/partials/cloudflare-one/email-security/deployment/mx-deployment-prerequisites.mdx

Co-authored-by: marciocloudflare <[email protected]>

---------

Co-authored-by: marciocloudflare <[email protected]>

Author: 2 people

src/content/partials/cloudflare-one/email-security/deployment/mx-deployment-prerequisites.mdx

@@ -8,6 +8,10 @@ To ensure changes made in this tutorial take effect quickly, update the Time to
 
 Changing the TTL value instructs DNS servers on how long to cache this value before requesting an update from the responsible nameserver. You need to change the TTL value before changing your MX records to Email Security. This will ensure that changes take effect quickly and can also be reverted quickly if needed. If your DNS manager does not allow for a TTL of five minutes, set it to the lowest possible setting.
 
+:::note
+Make TTL changes a few days before the production update, and wait at least as long as the old TTL values before making the update, since some senders might still be using the old cached values.
+:::
+
 To check your existing TTL, open a terminal window and run the following command against your domain:
 
 ```sh