I have a customer that I am in communication with directly and They are not receiving my Email invites to our customer conference and can't diagnose what the issue is.
From the given error from your second screenshot I think Marketo observed a soft bounce with error "554 5.4.5 [internal] Delivery failure".
Mail error 554 5.4.5 can be caused due to improper mail server configuration or anti-spam tool configuration.
I don't know what you're trying to show here. You have arbitrary lines highlighted from different logs.
The last ActLog detail reflects a clear DNS error, but you're hiding the domain so there's no way to check whether the error is still present now.
Sorry about that, I didn't include it for privacy purposes but the email domain is @bosheim-consultancy.nl
The mailserver for that domain does allow inbound connections at this time, but at the time the email was sent there's no reason to mistrust that MX connection error. That's the problem with trying to troubleshoot after the fact, even an hour later the DNS + MX may be stable.