I don't know about your country, but in mine (The Netherlands), it is a completely and utterly valid way to enter a contract.
Actually, you are free to enter a contract in any way possible. It is vormvrij (translated: form-free). Excluded is the purchase of a house, as far as I know. But for the rest, you are free to come to an agreement via WhatsApp, Facebook, email, or a scrawl on a piece of paper.
Because, without DKIM, a dishonest party can repudiate the email. Just as a written contract is superior to an oral contract, a non repudiable written contract is superior to a repudiable written contract.
A signed and scanned PDF is also commonly used, same as an old-school fax-based contract where you sign and send it back. But, yes, the DKIM definitely does not matter for contract purposes.
Actually, you are free to enter a contract in any way possible. It is vormvrij (translated: form-free). Excluded is the purchase of a house, as far as I know. But for the rest, you are free to come to an agreement via WhatsApp, Facebook, email, or a scrawl on a piece of paper.