Add greylisting capabilities for improvement to spam management

[d19dotca]

I'd like to request that we expose / add the capability to enable greylisting.

Greylisting is a technique that upon suspicious (or first-time mail server connections) the mail server tricks the sending server to think the receiving mail server is waiting/busy and cannot accept their connection. This should cause any legitimate mail sever to keep retrying into the future. Mail servers setup for spam only tend to not be configured correctly and won't retry, thus greylisting functionality can prevent spam delivery in some cases.

It's certainly not bulletproof by any means, but I found it useful in my previous mail server before moving to Cloudron. I think it'd be helpful to add this as an optional (probably off by default) feature in Cloudron.

[MooCloud_Matt]

The issue with Greylisting in email is that is super easy to just add delay on email delivery.
So need to be a really good consideration being that.

And all those features need a better cloudron UI to be able for users to release an email that is in graylist quarantine.

[d19dotca]

@moocloud_matt ...which is why it should be optional and not enabled by default.

Ideally too, there'd be a "whitelist" capability so that certain domains or IPs can completely skip all spam-related checks. That is also a feature request another user created a while back that I hope to see implemented too.

[d19dotca]

For reference, Haraka has a greylisting function built in: http://haraka.github.io/plugins/greylist/ - and it works with the DNSWL / whitelist which we should enable too. That way if an IP is on the DNSWL, it automatically skips any greylisting attributes.