Dorian Is sending a digitally signed email to Polly, with which key is Dorian signing this message and how is Poly validating It?

Signing with a public key provides no proof of identity because public keys are freely available to anyone - any party could produce the same result, entirely defeating the authentication purpose of a digital signature.

Encrypting with Polly's public key is the mechanism for confidentiality, not digital signing - it ensures only Polly can decrypt the message but does not prove the message originated from Dorian.

Dorian uses his private key - which only he possesses - to generate the digital signature, mathematically binding him to the message content. Polly then uses Dorian's publicly available public key to verify the signature and confirm the message originated from Dorian and was not altered in transit. This asymmetric process satisfies both authentication and non-repudiation requirements.

Dorian would have no access to Polly's private key - private keys must remain exclusively with their owner, and using another person's private key to sign a message would compromise that person's identity and is cryptographically invalid.