What is double DKIM signing and when is it necessary for email authentication?

Email marketer from SocketLabs explains DKIM signing and how it works. They say that DKIM is a way to claim responsibility for a message, allowing mail servers to verify that a message was truly sent from your domain. SocketLabs does not refer to 'double DKIM signing'

Marketer from Email Geeks explains they don’t need to bother you with the FBL set up if they are signing with THEIR DKIM domain. If they don’t, then they need to bother you.

Email marketer from Reddit answers a question about if you need to set up DKIM records for every subdomain used in email sending. They explain that as long as a valid DKIM record exists for the domain or a parent domain, that is sufficient. If the ESP signs with their own DKIM, its not your problem and you can't influence that, and whether that is done or not has no affect on whether you need to set up DKIM records for your sending domains.

Marketer from Email Geeks explains that signing with your domain means you accumulate the reputation and can take it with you if you move to a new ESP.

Email marketer from EasyDMARC explains that DKIM is a method to sign emails with a digital signature, and validate it with a public key on the DNS. It prevents spoofing and phishing attacks. While EasyDMARC has detail information, it does not include 'double DKIM signing'.

Email marketer from SparkPost outlines DKIM signing, a critical email authentication standard that allows receiving mail servers to verify the authenticity of incoming messages. This authentication ensures the message was sent by an authorized source and hasn't been altered in transit. The article does not specifically address the practice of 'double DKIM signing'.

Marketer from Email Geeks explains that double signing involves two signatures, presumably by two different domains.

Email marketer from Gmass explains how DKIM signing verifies the sender and domain of the message, ensuring it's not spam or phishing. By using DKIM signing, your emails are more likely to land in the inbox and not the spam folder. Does not explain double DKIM signing.

Email marketer from Mailjet explains that DKIM is a critical email authentication method that helps prevent spoofing and phishing attacks. It ensures that the email hasn't been tampered with during transit. The Mailjet documentation does not discuss the term 'double DKIM signing'.

Email marketer from Sendinblue explains that DKIM improves email deliverability by authenticating the sender's identity and protecting against spoofing. It helps build trust with ISPs and improve inbox placement. Information about 'double DKIM signing' is not mentioned.

Email marketer from StackOverflow answers a question about SPF and DKIM setup suggesting to check the headers after sending an email. If they are failing suggests looking at your DNS settings.

Expert from Email Geeks defines over signing as when you sign some headers twice to prevent DKIM replay attacks against your domain.

Expert from Word to the Wise discusses DKIM authentication failure, explaining that DKIM, along with SPF and DMARC, is used to authenticate email and prevent spoofing. When DKIM fails, it could be due to various reasons, such as incorrect DNS records or tampering with the email content during transit. If there is a forwarding situation, a 'double DKIM' record can sign. This is sometimes referred to as double DKIM signing, which ensures that no matter what happens to the email in transit, at least one valid DKIM signature survives to authenticate it.

Expert from Spam Resource explains that having multiple DKIM signatures can be helpful if you're using different email sending services or if you want to ensure that at least one signature remains valid even if one of your sending paths is compromised. It is important to get the syntax exactly correct though

Expert from Email Geeks explains that you don't need them to double DKIM sign for you as double signing is for their benefit. As long as they're signing with your domain, she wouldn't worry about it.

Documentation from Cloudflare explains what DKIM signing is, and how to validate a DKIM key. They specify that with DKIM, a sending mail server uses a private key to encrypt the message header. Receiving mail servers then use a public key published in the domain's DNS records to decrypt the header. This confirms the message's authenticity and verifies that it wasn't altered during transit. This doc doesnt refer to multiple or double DKIM signing.

Documentation from Google explains that DKIM signing adds a digital signature to outbound email messages. This signature is used by receiving mail servers to verify that the message wasn't altered during transit and that it truly came from the domain it claims to be from. They do not explicitly mention 'double DKIM signing' but rather standard DKIM practices.

Documentation from Microsoft emphasizes that DKIM is essential for authenticating outbound email. It allows recipient mail systems to verify the message's integrity and authenticity, reducing the risk of phishing and spoofing. Does not specifically cover 'double DKIM signing' but highlights DKIM's role.

Documentation from AuthSMTP answers explains that if the message gets altered during forwarding, the DKIM signature can be broken. They suggest a feature of 'Domain Signatures' which allows multiple DKIM records to sign. When a message is signed by more than one DKIM record, this is sometimes referred to as double DKIM signing, which ensures that no matter what happens to the email in transit, at least one valid DKIM signature survives to authenticate it.

Documentation from RFC6376 specifies the DKIM (DomainKeys Identified Mail) protocol, which allows a sender to digitally sign their email messages. This helps recipients verify the authenticity of the email and ensure that it hasn't been tampered with in transit. The RFC does not include any material on double DKIM signing.