Propagation is the process of updating a domain with new nameserver (DNS) information across all the servers and computers in the world via the internet. Until the propagation process is complete, a domain and its emails will not work consistently.
For more detailed information on how propagation of DNS records works, please read the following article:
Do I Have to Worry About Propagation?
You only have to worry about propagation when updating your nameservers or when you change a DNS record in your DNS zone file.
How Long is the Waiting Period?
Nameserver changes can typically take 0 to 24 hours to take effect, but they are known to take as long as 48 hours to go into full effect.
DNS zone record changes such as A, MX and CNAME records can typically take 0 to 4 hours to resolve but are known to take as long as 8 hours to fully propagate.
Can Propagation be Expedited?
Nothing can be done to expedite the propagation process; however, it is recommended that you plan for propagation ahead of time to minimize the effects of down time.
See the related article below for more information:
How Do I Know When Propagation is Complete?
WHOIS, ping and traceroute tests cannot indicate when the propagation process is complete. The only way to be sure is to wait the maximum time which would be up to 48 hours from the time that the DNS record was updated.
This means that if the nameservers are pointed to HostGator, then the DNS records must be updated at the same HostGator server. If the nameservers are pointed to another registrar, then the DNS records can only be updated at that domain's registrar.