If emails are a part of your core business, you should have a business continuity plan for this situation. That may be to fall back to another service, or even

If emails are a part of your core business, you should have a business continuity plan for this situation. That may be to fall back to another service, or even a temporary in-house server. Of course, if it's that important, then you'd be foolish to not have invested in a better solution, or to not have a proper SLA with a third party.