Setting up a DKIM record alone does not enable DKIM signatures depending on the email provider. The possible causes are an issue with the DKIM record or the email provider not supporting the DKIM signature by default.
First, verify that there is a proper DKIM record and validate it by using a third-party tool. If there is no issue with the DKIM record, check with the email service provider as most possibly they have not enabled the DKIM signature.
Read more about Cloudflare DKIM here.
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