You said the magic letters - SLA. This is why some companies pay big bucks for outsourced email. They guarantee a % uptime and if the provider doesn't meet it, I believe that you have legal grounds to sue for damages and disruption to business.
Honestly, if email is _that_ critical to you, you're going to have to pay for it.
They usually offer a money-back guarantee if they fail to meet the % uptime, but I don't know of any provider who would absolutely guarantee uptime and risk a lawsuit for not complying. I'm curious to know if there are some like that.
Honestly, if email is _that_ critical to you, you're going to have to pay for it.