After conducting tests with 06 different email accounts and domains, hosted by different providers:
03 inside Brazil: Hostgator, Locaweb and UolHost
03 outside Brazil: 01 ResellerClub and 02 on AWS LightSail
The solution is: GET OUT OF BRAZILIAN PROVIDERS
Result:
Inside Brazil 0 x Outside Brazil 3
It is a sad reality that had to be proven with a few hours of work.
It remains to propose to client to change provider or request an upgrade on the HostGator server (just a dream).
Thanks @DMehaffy for his last words!