Not sure I understand the question. I have to manage a domain and have a basic understanding of how DNS works, that is unavoidable. DNS is a bit complicated t

Not sure I understand the question. I have to manage a domain and have a basic understanding of how DNS works, that is unavoidable. DNS is a bit complicated to describe in short reply but on a very high level: say I register a domain example.com. Registrars usually give you an interface to manage DNS where you can set A records (association with IP), you basically put there 2-5 IP addresses of the DNS servers serving your domain. You can also setup MX records that are used to resolve mail servers for your domain. Setting up fastmail is simple: you tell them that you want them to serve mail for example.com and setup couple of MX records pointing to the Fastmail servers (they give you the full host names).