An email being filtered to a quarantine or bulk mail folder happens after the recipient mail server has accepted message. Once an email has been accepted by a mail server, it is impossible to tell where it went or what happened to it. Note that this is true of any mail sent by any system on the Internet.
Every mail server has configurable filters that determine how received mail will be handled. The mail server administrator should be able to adjust those filters to ensure delivery of emails based on their business standards, or there may even be end-user-configurable controls that can accomplish the same thing.
If test mailings you are sending to yourself or your colleagues are being filtered to a quarantine or bulk mail folder, you should consider asking your email administrator to whitelist Marketo’s IP ranges. They can be found here.