It has many potential uses, but within DKIM itself, it's an expansion socket.
Keep in mind that there are IANA registries for the tag names in both
the signatures and the key records. If you want to try something new,
just pick a tag name or two and have at it. RFCs 6541 and 6651 have
recently added signature tags.
I would think that i= would be a poor tag to use since a lot of people
already have opinions about what it means or doesn't mean.
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