So no mail flow restrictions on user mailboxes.
Hence, it is the default receive connector on Edge server that is not allowing the connection to your domain.
This is happening while sending to external or receiving from external mails?
check the mailboxes are enabled for below parameter yes disable the same.
The external sender > Is that sender sending an email to a distribution group or a user mailbox?
I assume you have an edge subscription set up?
Is this issue limited to a single external domain or all external emails (domains) are affected? Is this issue w.r.t to a single Mailbox user or multiple?
This issue is limited to one single external domain as far as I know.
Can you tell me which properties I can output for get-mailbox?
Is only one mailbox affected or all of them cant receive emails from this specific external sender?
Issue applies to multipe mailboxes.