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It's based on anti-spam provider, not technically the email provider (though they may be the same company).
In this case I think it's Symantec's security cloud, but it could be any of them -- the prefix is a cosmetic setting made by the IT admin.
You can't control whether recipients' servers will mark up your emails: as long as they can detect the Marketo origin (easily done) they'll assume they're marketing, even if privately you know you're using Marketo for emails outside of the marketing realm.
Thank you for explaining more thoroughly, Sanford!