This post answers the question "how do I host my email at one provider and my web site at another?". In particular it shows how to set up DNS at your domain name registrar, email at Fastmail and web hosting at WebFaction.
Most of the how to DNS configuration guides on the web show how to point to a single hosting provider for web, email and other services. I'm awkward and wanted to separate my email from web hosting. So by a little trial and error I came up with this set up for my personal domain.
First at your domain registrar (I use hover) set the domain namervers to Fastmail, I've used 2 of their name servers.
This delegates control of all DNS activity for the domain to Fastmail. Without any other changes this will allow Fastmail to manage email and serve a limited web site from their hosting option. I choose to host my web site at WebFaction and so to delegate this from Fastmail we use their domain configuration dashboard and set up DNS 'A' records to point to my WebFaction host.
The final step is to configure WebFaction to accept and serve your domain. If they don't recognise incoming web traffic you won't be able to server a web site on your domain. Here's mine set up in domains section of the WebFaction control panel.