IP Address

There isn't a specific location to find this in Marketo.  You might check for an IP address here:
http://www.kitterman.com/spf/validate.html

Alternatively, I'd contact Marketo Support to get a specific IP address.

You can also find the IP address in the header of a Marketo email that you send to yourself.  It will be the X-Originating-IP.

This article provides an up-to-date list of IP addresses Marketo uses to send emails:

https://community.marketo.com/MarketoArticle?id=kA050000000KypECAS