Long mails in export mboxes get cut off at random points

Originally reported https://progress.opensuse.org/issues/95756

The mboxes of long mailing list archives get cut off at random lengths, for seemingly no reason. It appears the length is non-deterministic. You can check archives like https://lists.opensuse.org/archives/list/factory@lists.opensuse.org/export/factory@lists.opensuse.org-2021-07.mbox.gz?start=2021-07-01&end=2021-08-01 (and scroll down to the bottom) to see the issue