Repeated Blacklisting / IP reputation

That would be very useful. Many of those small lists return 'Unknown
user' rather than an actual blacklist message. A url where one could
get reason (meaning headers) for the block would be even better. If
they don't admit that it's a block, it's hard to do much more than tell
the user to contact the recipient via some other channel and have *them*
contact their support system.