To allow Trustpilot to send emails on your behalf you will have to add it to your SPF record. The SPF record mechanism used for Trustpilot is shown below.
NOTE: Trustpilot does not support DKIM signing at this point. Please see the following article for more information on SPF and DKIM.
Trustpilot do not seem to support DKIM at this point. Please contact them for more information.
Create a free OnDMARC account to test your configuration.