All domains will have at least two DNS servers associated with the domain. Many domains will have more than two. The DNS data on all servers for a domain need to be identical. If you are seeing an issue where sometimes a query returns the proper result and sometimes it does not, it could be an issue of synchronization between the DNS servers for a domain. To investigate this, you can send a query to a specific DNS server.
For example, if we look at the Name Servers for Valimail.com, we see:
dig valimail.com NS +short
If we want to send a query to a specific DNS server in the above list, you can use the ‘@’ option for the Dig command:
dig www.valimail.com TXT @ns-1144.awsdns-15.org. +short
If you run the same query against all servers individually, you can see exactly what data each has for a specific DNS name.
One other note on queries to these servers directly, there is no DNS caching