Last two days I had trouble to get connected to iCloud. While I was able to receive emails, I was not able to send some. The SMTP Server form apple has not accepted any emails. After working with Apple Support with a kind of try-and-error, I remembered that I had installed GPG suite 2 days before. While the mail plugin was not enabled, the situation was back to normal after I uninstalled the GPG Suite.
This has nothing to do with NK’s - it might be just the case that you have a Mac and try to use NK’s with GnuPGP . I am a strong believer of the fact that the errors often sits in front of the keyboard, so it would be great to hear if someone else has had the same trouble ( and a diffrent solution than just de-install ?)