Can't receive email

I am new on Virtualmin and generaly on Linux. I have a server with CenOS 7.5 minimal installation. I am trying to make it work as mail server.
I installed virtualmin and i created a virtual server, then I made user and I sent an email to my gmail account. Everything is fine till now. When I sent back an email to the new server on gmail looked like it was sent but I never received it on the new user. Any ideas