You won't be able to send emails from any external domain using Marketo as a default. Marketo uses their own MX records to perform the send for you. You can however, specify these domains in your SPF/DKIM. With that being said, I'm not sure what pro/cons you'll experience.
Jim, thanks for replying.
I think i didn't express myself correctly. I understand Marketo will use their MX, i meant my "from" emails.
I'd use, for example, firstname.lastname@example.org and the "from" email on my campaigns.
The question is, do you think I'll have any problem with that, since it doesn't use a traditional TLD or ccTLD?
You guys are misusing "MX". MX solely refers to incoming mail.
Marketo will use an envelope sender @*.mktomail.com -- unless you have specifically purchased the branded sender domain option.
From/Reply-To: headers can use any example.tld. It doesn't matter whether it's a ccTLD, a new "vertical" TLD like .lawyer, or a wholly owned gTLD.