Chris is right here. Marketo is still doing all the sending behind the scenes. Regardless of the tool you are using to send, your overall spam/deliverability rating is always tracked back to your sending domain/subdomain. Ex:// @email.mycompany.com. So regardless of the tool you're using, if you are sending from that domain and getting marked as spam, that domain will always suffer the consequences. And again, I believe deliverability issues only affect the subdomain, not the root domain. Maybe someone else can confirm my understanding of this?
Another option may be to create a new subdomain for this scenario (so you don't risk blacklisting your existing sending domain), though I know deliverability can be an issue when you start using a new domain as some IP warming generally needs to take place. So in short, its not the tool that gets blacklisted, its your sending domain.