To authorize Gandimail (Gandi.net's mail service) to send emails on your behalf you will have to include it in your SPF record. The SPF record mechanism used by them is shown below.

include:_mailcust.gandi.net 

For more information on SPF please click on the button below.

Please see below the article on how to configure DKIM on their platform

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Create a free OnDMARC account to test your configuration.