I am having trouble with DKIM authentication when sending emails from my domain through a relay server. Here is a brief description of the problem and my current configuration. Looking forward to your support and experience sharing.

When sending an email from admin@yourdomain.com via the relay server mx5.relayserver.com (IP: 203.0.113.45) to recipient@gmail.com, the email is marked as spam by Gmail with the error:

550 5.7.1 [203.0.113.45 12] Gmail has detected that this message is 5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail, 5.7.1 this message has been blocked. For more information, go to 5.7.1 https://support.google.com/mail/?p=UnsolicitedMessageError d9443c01a7336-20d1804392esi54129835ad.194 - gsmtp

Authentication-Results: spool.domain.vn; iprev=pass (103.126.176.38); spf=pass smtp.mailfrom="admin@yourdomain.com"; dkim=none

Authentication-Results: smtp.domain.vn; dmarc=passed (v=DMARC1 p=quarantine rua=mailto:dmarc-reports@yourdomain.com ruf=mailto:dmarc-reports@yourdomain.com fo=1 pct=100); spf=pass smtp.mailfrom="admin@yourdomain.com"

Although SPF, DMARC, and Blacklist are all Passed, DKIM is not properly authenticated (dkim=none).

The above information has been edited to avoid revealing my email information. Thanks everyone for reading and supporting!