Email is very much not deniable in practice. Almost all email is sent via third parties who create records that any court will accept as true. DKIM doesn't significantly change that.

Remember that I sued Lovecruft for defamation, and she didn't even try to deny sending incriminating messages on _Pond_, a extremely secure crypto-nerd anonymous messaging platformed designed for deniability.

Courts almost always accept electronic records as true by default.