K-9 Mail 5.600 moves messages to the Drafts folder when there was a permanent error when trying to send the message. To find out what went wrong exactly you’ll have to enable debug logging and fetch the log from the device. Unfortunately, this process is quite involved. See https://github.com/k9mail/k-9/wiki/LoggingErrors
K-9 Mail 5.7xx won’t automatically move the message to the Drafts folder and should display an error notification giving an indication of what went wrong. Consider switching to this version. See How do I become a beta tester?