A domain is the name of the website, or in this case, email server. IE bleepingcomputer.com or zoho.com. Think of DNS being like telephone information (in the US 411) but instead of people's names and their phone numbers, it gives information about websites(domains) and their matching IP addresses. The domain > IP record is called the "A" record. The domain > mail server is called the MX record. Below is the MX record for zoho.com or see here for easier to read format:
Pref Hostname IP Address TTL 10 smtpin.zoho.com 22.214.171.124 5 min Blacklist Check SMTP Test 20 smtpin2.zoho.com 126.96.36.199 5 min Blacklist Check SMTP Test 50 mx3.zoho.com 188.8.131.52 5 min Blacklist Check SMTP Test
Running some simple tests, I see the the domain is missing the "demarc" record and the SMTP server failed to connect. What website are you trying to register for? The domain DOES have a MX record, but it's missing some parts the website might be checking to make sure it's valid. Many websites will have systems that check if a email is fake, and sometimes free email addresses can set these systems off by accident.
Edited by Trikein, 12 December 2016 - 06:03 PM.