SMTP emails not getting signed / DKIM

Hi,

My DKIM and SPF are well configured in Cloudflare.

I use WP-SMTP to send emails from WP.
I can send emails using:
– SendInBlue with API
– MXROUTE via SMTP

The test email sent from from WP-SMTP show perfect signed email in both cases so all is good.

However, when using CF7 for my contact forms:
– If using SendInBlue/API in WP-SMTP=> no problem
– If using MXROUTE/SMTP => Emails are being delivered but NOT SIGNED. So they end up in the spam folder.

I tried using WPFORMS with same results.

Would somebody know why emails don not get signed when sent via a form plugin?
Thank you