First of all, I want to apologise to @sahsanu and everyone who tried to help me with my email problem.
I want to say that the whole problem was mine, that it had nothing to do with HestiaCP and the whole environment.
While I was making queries to the forum, I was doing NS correction tests and that’s why the information was so confusing. At the beginning everything was going through Cloudflare and that meant that not everything was as it should be. Once corrected and gathering information from all over the net, I solved the problem that was mine and mine alone.
I have to say that the error was that I had misspelled records and being obsessed with that (along with the sense of urgency on the part of the client) I was not able to see them.
Thank you all very much for your help.