Yes, this is a VERY serious matter. I've experienced it and gone to a lot of effort to capture the evidence for DH Support, only to receive the disappointing news that they can "only fix bugs that are 100% reproducable". But I do feel they will see the error of this policy if they get enough feedback.
I urge you to submit a ticket.
In my case, I was mailbombed to over-quota, which prangs the mailbox by starving it of workspace.
DH said that because the logs said the lost mail was delivered by the server, they couldn't do anything about my report that it was never actually received by the mailbox. I was really disappointed that they didn't take this disaterous security hole more seriously.