Don't Prepare() the message in SmtpClient.Send*() if the message is DKIM-signed
Don't Prepare() the message in SmtpClient.Send*() if the message is DKIM-signed

If any of the following headers are included in the message,
do NOT prepare the message:

DomainKey-Signature
DKIM-Signature
ARC-Seal

(ARC-Seal infers ARC-Message-Signature and other ARC headers, so no
need to check for them explicitly)

jstedfast/MailKit,jamie-dainton/MailKit