A difference in what people see.
THE LIKELY CAUSES
A hosts file entry on your machine, left from testing a migration Your own DNS cache holding a different answer A different network with a different resolver A firewall blocking their address rather than yours
THE HOSTS FILE
The most common and most self-inflicted. An entry added to test a new server overrides DNS entirely, permanently, until removed.
You see the new server; nobody else does. Or the reverse.
Check it before anything else if you have migrated recently.
YOUR OWN CACHE
Flush it, or test from mobile data with wifi off.
IF IT IS A SPECIFIC CUSTOMER
Their network, their resolver, or their address being blocked.
Ask them to try mobile data. If it works there, the problem is their network.
IF IT IS EVERY CUSTOMER BUT NOT YOU
Almost certainly a hosts file entry or a stale local cache.
THE TEST THAT SETTLES IT
Query your authoritative nameservers from a lookup tool. That is what the world sees, regardless of what your machine believes.
