If you send email through Xs4all, receiving servers decide whether to trust those messages by checking SPF, DKIM and DMARC for your domain. Under DMARC (RFC 9989), at least one of SPF or DKIM must pass and align with the domain in your From address, which usually means authorising Xs4all in your SPF record and publishing the DKIM key it gives you, so you can move your DMARC policy towards quarantine or reject without breaking those emails.
The exact include, selector and key values depend on your Xs4all account, so we don't guess them here. The EmailAuth AI Agent reads your domain's live DNS, tells you what's missing for Xs4all, and gives you ready-to-paste records, or walks you through each step. Start with your domain below, or book a free call with an EmailAuth engineer.