Here is how to configure your records for email to work when a Backup MX (AKA Mail Bagging or MX Backup) Server is involved and the Mail Bagging process:

1. User sends an email to 'user@abc.com' (a mailbox hosted by your mail server)
2. Their mail server looks up the MX Records for 'example.com' and finds two:
   IP: x.x.x.x Weight: 10
   IP: y.y.y.y Weight: 20Their mail server first attempts to connect to: x.x.x.x
3. Connection fails, which could be caused by server failure, server overload, power outage, etc.
4. They try to connect to the secondary MX record: y.y.y.y
5. They successfully connect to this server.
6. Email transmission begins, and the Backup MX Server receives the email into its spool.
7. Since there are no existing local domains on this server, the Backup MX Server stores this email in its spool.
8. Based on Retry Attempts, the Backup Mail Server will continue to try and make connections to your Primary Mail Server.
    Backup Mail Server (aka Backup MX) will only make retry attempts.
    The Backup MX Server (aka MX Backup Server) does not send a Bounce Message to the originator saying that it could not deliver the message before your Primary Server is back online.
9. If your Primary Mail Server comes back online before the final Retry Attempt (which is up to 20 days for Shared Mail Bagging Plans) then all spooled or backup emails will start delivering to your primary mail server.