It would take a book to fully explore all the issues and possibilities but it still comes down to - how much money you have to spend on the project.
I use to work with High Performance Clusters and HA Linux Oracle SAP global deployments with TB's of data - this issue would arise daily for us in those environments.
Ideally, I would buy two cards that had two ports (Gb).
I would then channel bond 2 of the ports across the cards, for 2 separate bondings.
When exploring these items, you want to have multiple channel paths to make sure copies, rsync's, flashing occurs differently as compared to the "production" path.
Have your network backup occur on a different ethernet device.
Now, a mail server isn't a HPC compute node - I understand that.