Issues with just one email address not receiving mail outside the domain

[RaresKool]

I have been trying to figure out why one particular address, let's call it [email protected].
It can receive emails from other addresses @mydomain.tld, but it can not receive mail from other mail services (i.e. gmail.com, yahoo.com, etc.)
On the other hand it can send mails to both addresses @mydomain.tld and others such as the ones mentioned above.
All other addresses @mydomain.tld can both send and receive mail from outside services.

Looking forward for suggestions.
Thanks.