Hi,
I have a problem with the email sent from my server (with Postfix + opendkim) and I receive the error “Your DKIM signature is not valid”.
I tested the configuration with a lot of tool, I switched to a 2048 key, I changed the key from the default but the problem still exist.
Cloudflare does not perform any DKIM signing. If your DKIM record is published correctly in your Cloudflare DNS, there isn’t anything else that can be accomplished in the Cloudflare forums.
I recommend the dmarcian forums for assistance on DMARC, DKIM and SPF. I am active there as well as here which means I can guarantee a response if you post there.