[TenFour]

I just came across this unanswered question. The problem with that configuration is that your incoming email could end up at either Runbox or Fastmail. Chances are most of the incoming email will end up at the service with the highest priority, but not all of it. One simple way to deal with this issue is to have your email forwarded to two different email providers. Some domain registrars have this capability built in, and there are services like POBox.com (reliable) that specialize in this. That way all of your incoming email will appear in two different places and you can set both places to send from a third party SMTP service if you want to! However, to me the more complicated you make your email system the more problems you encounter. For example, two different services have two sets of SPAM rules, and sometimes an email that gets whitelisted in one is not in the other and you begin to lose emails. Stuff like that. Both Runbox and Fastmail have been reliable and I suspect it is unlikely to "lose" any emails if one of them is down briefly. The way email works is typically the sending server will retry for awhile, and if an email doesn't go through for some reason the person at the other end will get a message that will prompt them to retry at a later date and/or contact you another way. I use Office 365 email at work and this happens surprisingly often with email sent to smaller email providers, but very rarely to a current and working email address at a major provider.