When sending a campaign, the package will create Send models for each mail to be sent. A Send has a property sent_at that stores the date time of when an email was actually sent. If that attribute is null the email has not yet been sent.
If you experience problems while sending, and the state of your queues is been lost, you should dispatch a SendMailJob for each Send that has sent_at set to null.
You can run the above code by executing this command:
php artisan email-campaigns:retry-pending-sends
Should, for any reason, two jobs for the same Send be scheduled, it is highly likely that only one mail will be sent. After a SendMailJob has sent an email it will update sent_at with the current timestamp. The job will not send a mail for a SendMail whose sent_at is not set to null.