There are several helpful websites that allow you to look up the DNS records of a domain. This is particularly useful for clarifying who your email provider is and what instructions are needed to direct email traffic, i.e. MX records.

MXToolBox - Easily look up your domain's current MX records. The user interface is clean and focused on email records.

Central Ops - Find all the DNS information about a domain, including MX records and name servers. It can be more difficult to read the results compared to a search on MXToolBox.

What's my DNS - Tracks propagation of DNS records.