I think it's too confusing to create a new category for this. It's either `ISEMAIL_RFC5321` or `ISEMAIL_DEPREC`.

I chose `ISEMAIL_RFC5321` but I'd be happy to change it to `ISEMAIL_DEPREC`

I think I'd also add a new RFC reference to the text in RFC 5952: "all implementations must accept and be able to handle any legitimate RFC 4291 format" and add it to the diagnosis `ISEMAIL_RFC5321_IPV6DEPRECATED`. This will make it clear that although this format is deprecated, it is valid and should be allowed if encountered.