Debian – Exim installed, can send mail but not receive any

I am trying to set up the mail service on my server. I installed exim4 and configured it.
I can send emails to any email address, send one from a user to another but not receive any.

When I try to send one from gmail I get a mail from gmail daemon with the subject: Delivery Status Notification (Failure) stating Recipient address rejected: User unknown in relay recipient table

The user exists for sure because I replied to the mail I first sent from my server.

My MX lookup:

example.org mail is handled by 10 mx2.example.org
example.org mail is handled by 10 mx1.example.org

Any idea on what is going wrong?

Thank you in advance