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DKIM i= tag

The optional tag in a DKIM signature naming the agent or user the signer acts for; its domain must equal or be a subdomain of d=.

What it means

i= is the Agent or User Identifier (AUID). It looks like an email address, such as i=newsletters@mail.example.com, though the local part can be empty and need not be a real mailbox. When it is absent, it defaults to @ followed by the d= domain.

RFC 6376 leaves its meaning to the signer and warns verifiers against relying on it. DMARC ignores i= and aligns on d= only. A key record with the flag t=s forbids i= from naming a subdomain of d=.

In ARC headers the letter i means something else entirely: the instance number of the ARC set.

Example

i=@mail.example.com

On this site

Sources

What this definition rests on. The wording is ours; the facts are theirs.

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