Leaving the book out of this
The issue appears to be:
Calibre
thinks the mail was sent.
AND
The mail went somewhere else (wrong destination or Users PC SPAM trap:
Outgoing or Incoming)
Or
The mail was trapped by the destinations ISP (on line SPAM trap)
Check all logs.
Your PC A/V should have an activity log (in case it trapped suspected malware originated mail)
Your ISP should have a Graymail log