In life, most of the times, things are neither black nor white, but grey. And so it happens in e-mailing.
You have probably heard of the terms Whitelisting and Blacklisting. Both are special kind of lists you can have in your e-mail client (or even in the server). There, you note down e-mail addresses of people who you always trust or who you don't want to hear of, respectively.
In this way you keep a couple of lists to separate the good from the evil. When you receive an e-mail which is clearly spam, you add the sender to the blacklist, so that you will never receive anything from her. On the other hand, you add to your Whitelist the e-mail addresses of friends, colleagues, and everyone who is always welcome to your inbox.
These kinds of listings are very ineffective for several reasons, mainly because:
1. - It's a pain to keep them updated.
2. - Spammers generally use random generated sender addresses (name and domain), so the effectiveness of Blacklists is very limited, because each time the same spammer could be anyone.
3. - A lot of viruses and e-mail harvesting malware use the infected user's e-mail address as the sender for their e-mail. So, if one of your trusted senders is infected you will receive a lot of spam or viruses and your Whitelist will do nothing for you in this case.
So, what can we do?
There is a mid-term solution which is neither white nor black: it's grey! It's called Greylisting. It works this way: the first time someone send an e-mail to your server she gets banned with a transient error. Legitimate servers always try to deliver again e-mail several minutes later, and this second time the receiving server will let the e-mail go in. In addition it will put the sender in a greylist for a couple of days, and it will be trusted during this period. This works extremely well with spam because most of spam programs (and a lot of other not well designed bulk e-mail programs) just do "fire and forget", and if e-mail is not delivered at first chance they will not retry later.
So, again, grey is always better than black or white, and with such a simple trick you get rid of a huge percentage of your spam.
Of course, MAILCast supports grey listing retrying so that you will never miss the chance of delivering your law-compliant e-mail to your customers.