Not all emails are being forwarded

I get mail from established providers, but not from myself.

@ Gmail => myDomain = forwarded to gmail
@ myDomain => Gmail = forwarded to gmail

@ hotmail => myDomain = forwarded to gmail
@ myDomain => outlook = forwarded to gmail

@ ISP => myDomain = forwarded to gmail

etc etc

@ myDomain => myDomain = does not arrive.

When I say “forwarded to gmail” it’s usually in the inbox, but occasionally in the spam folder. However, from person1@myDomain to person2@myDomain/secondDomain there is nothing. Not even a receipt in the Email Routing summary