We wish we could provide you with one static IP address. We are deployed into the cloud and utilize over 200 servers to process emails and user requests. Unfortunately, at this point, it would be difficult to give you a single IP address to whitelist. We could give you a pretty expansive range, but I'm afraid that is not going to be any more secure for you.
An alternative for you would be to get your own http://sendgrid.com account for your company which you can then use exclusively on your Tout account. If that is a route you'd like to explore, we're happy to talk you through it, please contact us at firstname.lastname@example.org to learn more.