Emails on Hostname send but not receive ...

Hello,

I have 2 domain names, 1 set as hostname the other as account.
I've created an email account on each of them

The email account on hostname sends emails but it does not receive.
The email account on hosted domain name is working ok.

If I rename the hostname to something different that first domain name, the email account starts to work correctly.

What I am doing wrong?

Thank you!
Kind regards,

be sure that hostname isn't set to the domain name as it needs to be set as subdomain.

example
srv1.domain.com