You can either go with an SMTP or DKIM setup. Both of which have their pros and cons.
SMTP: Is a free service and all you need to do is send our support team (firstname.lastname@example.org) an email with your SMTP information (host, port, username, etc..). However, this configuration will not be able to handle bounce back emails. You will have to manage this on your end.
DKIM: Offers additional security to emails and can handle bounce back emails. However, this is a paid service. You will need to contact your Customer Success Representative (email@example.com) for more information on the fees.
Thanks for the swift response.
So there is a way to send from the customer domain; but is there also a way not to mail at all? So grab the signing url from the API and distribute it in some other way to the signing user? (e.g form a mobile app, or through a push message?