As I mentioned in another thread, about the same topic:

First Zendesk dismissed the report. Then as hackermondev (the hunter) contacted Zendesk's customers, the issue "magically" becomes relevant again, so they reopen the report and boss the hunter around to not disclose it with the affected parties.

Hackermondev did the morally right thing - from his PoV it was clear that Zendesk wasn't giving a flying fuck, so he contacted the affected parties.

All this "ackshyually it falls outside the scope of the hunt" boils down to a "not our problem lol". When you know that your services/goods have a flaw caused by a third party not doing the right thing (mail servers not dropping spoofed mails), and you can reasonably solve the flaw through your craft, not doing so is irresponsible. Doubly true if it the flaw is related to security, as in this case.

I'm glad that Zendesk likely lost way more than the 2k that they would've paid hackermondev for the hunt. And also that hackermondev got many times over that value from the affected companies.

I work with Zendesk and this doesn’t surprise me at all - the product is janky and they’re much more interested in sales and squeezing a few more $$s from their clients than improving it or fixing issues