Ok, thanks for the update. From what you describe that seems either a corruption in the mbox itself or a bug in thunderbird which failed to process correct some emails. In any case not a downstream issue and if you delete the email we don't have a way to debug now so closing the report.
** Changed in: thunderbird (Ubuntu) Status: Incomplete => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2064039 Title: TB Snap fails to import/display large mbox-file completely To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/thunderbird/+bug/2064039/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs