Forward spam to a dedicated spamcop email
In the administration interface, there should be an option to report spam automatically to spamcop (or to any organization offering a collaborative DNSL).
[x] Forward emails marked spam to spamcop.
Of course, it would require mailbox.org to open a paid account at spamcop, but prices are pretty low. It would allow Mailbox.org to have a dedicated reporting email. To report a spam, just forward it to this dedicated email. This is probably the easiest way to report a spam.
When user clicks on button "spam", it would forward the email automatically to Spamcop, and add the MX to Spamcop DNSBL.
If you also filter incoming email connections with Spamcop DNSBL, this would make a quite reponsive solution.
By the way, no other email provider is proposing this kind of collaborative service.