Hi @Jhon Fredy
From your description, the internal AD domain (internaldomain.local) doesn't match the external mail domain (externaldomain.com) in your environment.
I suppose that emails sending via OWA worked fine and the only emails sent via Authenticated SMTP were marked as spam.
Do other recipient domains also reject the emails or it is just the specific recipient?
According to this link: Message tracking
If Message-ID isn't set in the SMTP application, Exchange server would set it in the form of <guid@serverFQDN>.
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