How to sign emails with DKIM and mimekit package?

See more:

C#

ASP.NET4

mail

I want to sign my emails - generated by a MailMessage class - with a DKIM signature from MimeKit - but got the error "Private key not found." in the line "

DkimSigner signer = new DkimSigner(stream, "my-site.dk", "dkim")

" And I don't know why :-( I have checked that the file with the private key exists.
I have searched and googlet everything until now - but with no success :-(

Therefore I kindly ask for help.

Best regards,
Michael

What I have tried:

I have tried to compose a signing with DKIM in this way:

var message = MimeMessage.CreateFromMailMessage(mail);

HeaderId[] headers = new HeaderId[] { HeaderId.From, HeaderId.Subject, HeaderId.Date };
DkimCanonicalizationAlgorithm headerAlgorithm = DkimCanonicalizationAlgorithm.Relaxed;
DkimCanonicalizationAlgorithm bodyAlgorithm = DkimCanonicalizationAlgorithm.Relaxed;

MemoryStream stream = new MemoryStream(File.ReadAllBytes("C:\\DKIM\\dkim.my-site.dk.pem"));
DkimSigner signer = new DkimSigner(stream, "my-site.dk", "dkim")
{
    SignatureAlgorithm = DkimSignatureAlgorithm.RsaSha256,
    AgentOrUserIdentifier = "@my-site.dk",
    QueryMethod = "dns/txt",
};