MX Backup Mail Service
If you are using an in-house mail server then you should protect it with an MX backup service.
This protects your messages against loss if for any reason your own in-house server is offline or connection is lost. If your in-house server or broadband connection is down, mail will re-route to the backup server which will queue the messages until your own server is back online, then transfer the automatically.
Note: To add the backup service to domain, you will need to be able to add a new MX record in your domain DNS records.