The text and reason for the failed mail delivery is shown in your attached image.
550 This message was classified as SPAM and may not be delivered
The receiving MTA has made a policy decision to refuse your message and has labeled it as spam. You will need to contact the postmaster at the destination if you disagree with their determination.
K-9 Mail isn’t doing that. Your email provider is sending you this message to let you know it wasn’t able to deliver a certain message. It doesn’t include the whole content because it’s a message you’ve sent. You should know what it says and ideally still have a copy of it in your Sent folder.