allow_nets based on RBL

[Tom Sommer]

This was brought up in 2014, and left without conclusion, so I thought 
it would be time to bump it :)

I would love a way to do allow_nets based on an RBL check, could this be 
added to the feature-list?

https://wiki2.dovecot.org/PasswordDatabase/ExtraFields/AllowNets

Thanks
--
Tom