I wanted to document a feature I have found useful with another email provider as a suggestion for Runbox.
Send a daily (or whatever frequency is required) spam report via email.
An email is generated that provides a list of emails (the list would contain the “From address” and “Subject heading”) i.e. a summary of emails that have been caught by the spam filter since the last report. This summary email would arrive in the Inbox along with other emails. I use Thunderbird via POP and this is useful in case an email has gone to the span filter that should not have (that I would not know about without logging into the Runbox web interface). I can then log into Runbox itself and flag the incorrect emails as non spam.