I had this issue too. Any email being sent to my company domain was being sent locally. This is (I believe) because in Plesk (yes it’s a Plesk issue).
So basically it’s going oh firstname.lastname@example.org is the registered user lets send any @company.com emails locally or something like that.
Anyway, I have had to fix this twice now and I did it by editing the /etc/postfix/main.cf file and commenting out the lines that started with “virtual”.