How They Work
Every domain has a DNS zone, a short list of records that map the name to the services behind it. The A record points the website at a server. The MX records, short for mail exchanger, point email at one. A sending server asks the domain's DNS "where does mail for you go," gets back the MX list, and connects to the first server on it.
gableexterior.example. MX 10 box.perfectsplice.com.
The number is a priority. When there is more than one record, the lowest number is tried first and the others are fallbacks. Most small businesses need exactly one.
Where It Goes Wrong
A domain moved to a new website host with the old MX records left pointing at the old mail provider, so the website works and the email quietly stops. Two MX records with the same priority pointing at two different providers, so some mail lands at each and nobody sees all of it. MX records pointing at a server that no longer exists, so every message to the business bounces and the business never knows, because the bounce goes to the sender.
How To Check It
Any DNS lookup tool, with the domain and the record type MX, shows what the world sees. The hostnames in the answer should belong to the provider the business is using today, and there should be no record pointing anywhere else. On a Mail-in-a-Box setup the records are generated by the mail server itself, so they cannot disagree with it.
The Foundation Connection
MX records are part of the DNS that holds the Digital Foundation together, and one of the cornerstones, professional email on the business's own domain, is only as reliable as they are.
