DNS propagation is the time it takes for any changes to your domain name’s DNS settings to take effect globally.
For example, you have just added new Name Servers to your domain name to point it to your new website hosting server where your website resides.
Because of propagation, not all visitors will be directed to your new website; some visitors will continue to see your old website until propagation is complete.
For example, the new Name Servers for your new Website might have already propagated in one geographic area, such as Europe, but not Australia. As a result, users in Europe will be able to see your new website, however Aussies may still be looking at your old website because the new Name Servers have not propagated down under, just yet.
As such it may a good idea to display a message on your old website by making necessary changes to your old hosting account saying that your new website is coming soon. Normally the DNS propagation is fully done within 24 to 48 hours.
You can check if your DNS records have propagated by using DNSnaut DNS propagation tool.
How quickly visitors can see your new website depends on where they’re based geographically, their internet service provider (ISP) and the TTL etc. Once propagation across the planet is complete, both your new website and email service will become fully functional across the globe.
Now you ask what TTL is. TTL stands for Time to Live which is a value you set in your DNS settings when adding or changing DNS records. It’s recommended that you enter a lower TTL value, such as, 300, so that any change made to your DNS records start propagating after 5 minutes.