In email marketing, a black list is a register of email accounts or IP addresses that are blocked as considered source of spam messages (which will thus be sent to the junk folder, or directly filtered out).
On the contrary, a white list is composed only by trusted contacts or IPs that will never be labeled as spam, because from them you receive only legitimate and approved mail.
That’s how spam filters work: when you keep on receiving unsolicited and spammy emails from an IP or an email account, it’s “blacklisted” by your incoming server. The action can be very strict – blocking all incoming messages – or less.
SpamHaus and DNSBL provide constantly updated series of blocked IPs (converted into domain names), making it very easy to verify if the protocol from which you’re mailing out is reliable (on a white list) or not.
Of course, maintaining a whitelist or a blacklist of single email addresses is hard work; that’s why a marketer normally uses these terms speaking of IPs.
In general, the blacklist idea is a good one: but unfortunately, being spam a big plague for the internet, the providers’ filters have been getting more and more severe: so it happens them to reject also legitimate emails.
In fact, from 10 to 15 percent of all your messages will be filtered out – that is, not even delivered – without you even knowing: which is of course a major issue for any email campaign. That is because if you send a mass email relying on a common SMTP provider may end up sharing an IP with a spammer – or even using a blacklisted one. And it doesn’t matter if you’re innocent and did it all right…
So the best way to avoid all this problem and ensure that you’re always whitelisted is to choose a professional SMTP service that will employ only monitored IPs.