public abstract MimePart Sign( MailboxAddress signer, DigestAlgorithm digestAlgo, Stream content, CancellationToken cancellationToken = default )
signer is null.
content is null.
|digestAlgo is out of range.
|The specified DigestAlgorithm is not supported by this context.
|The operation was canceled via the cancellation token.
|A signing certificate could not be found for signer.