You can bounce broken E-Mails to “force” a new fetch to zammad - especially in your case that should work.
If you do not want to do that, you can use the content of the E-Mail under /opt/zammad/var/spool/unprocessable_mail/
to add the E-Mails content to a manual ticket.
That’s how I’ve done it with 1 Ticket. 
Edit: After that you can delete the E-Mail from the zammad tmp-directory without any danger.
Edit2: Prior Zammad 6.0 those Emails where located within /opt/zammad/tmp/unprocessable_mail/
You can always see the latest version of commands in Zammads documentation:
https://docs.zammad.org/en/latest/admin/console/other-useful-commands.html#reprocess-unprocessable-mails