We use SendGrid to deliver email and they set the return path on any email that's delivered to your recipients to match our domain. This means that DKIM and SPF setup shouldn't be necessary and most mail clients will be transparent about the sender and include something like 'via Accredible' in the Fromfield.
If you need to whitelist our email sender information then please use the following information.
Default email sender: firstname.lastname@example.org
IPs that email may be delivered from: 188.8.131.52 and 184.108.40.206.
SPF record: include:sendgrid.net