A mailbox that sends email can now tell your code what happened to it. For each recipient, a mailbox.message.delivered event fires when their mail server accepts the message, and mailbox.message.failed fires when it bounces.
What you need to do. Nothing to keep working. To react, subscribe a webhook to mailbox.* or read the event stream. Match on messageId and actionId, since statuses can arrive before or after mailbox.message.sent.
In samva 0.25, MailboxEventType includes these events, so a switch over it sees them.