Is there any documentation about that Opens Sales Email where we must send email through MSI only (to share with business)? Or mb we have some features in Marketo that can help as catch send/open/click event in email which was send through SFDC?
You can’t detect interactions with email that Marketo doesn’t ever see or send.
So, by definition, if Opens Email or Opens Sales Email is logged, the email must be sent via Marketo.
There are tools (Seismic is one) that allow you track emails sent via SFDC — though not, to be clear, using native SFDC sends, but 3rd party packages — and the SFDC activities can then be seen in Marketo as part of the typical sync.
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