Thx for your reply,
AM, There is modification in some ways occurring during the email path to delivery.
Agree, yes I try out add some commands like include: not working…
AM, Have you verified that the correct key pairs are being used?
Yes, Tested with Dmarcian DKIM Records checker all ok.
AM, Is the DKIM signing done by Dynu outbound gateway, or something else beforehand?
Dynu DomainKeys Identified Mail (DKIM)
Step 2 : Save the private key on your email server for easy reference.
To solve my problem I change philosophy using the 2 DKIM key’s:
01 Client connect to the server using IMAP-SSL Port 993 (secure connection).
02 Dynu SMTP Relay using TLS Outgoing Port 587 (secure connection).
03 Dynu DKIM Public key2 are in place to control mail transfer (Secure).
Mains all communication IN and OUT the mail server is secure connected.
QST Do I still need the DKIM DMARC from my Synology mail server?
I don’t thinks so, maybe I am wrong tell me…
So I disable DKIM DMARC in the mail server and remove DKIM key1, DNS TXT Record.
From here I test Gmail again SPF DKIM DMARC resulting ALL PASS.
AM, It is working for now
Best regards Robert (PL)