A blacklist is a register of IP addresses that fail to satisfy criteria for ‘acceptable email’; this is often provoked by a large number of spam complaints. So ‘being blacklisted’ is a phrase no one wants to hear.
What do blacklists do?
A blacklist serves as a tool for ISPs to decide whether action needs to be taken – such as blocking further outgoing mail from a sender.
There are several of them too (Spamhaus, Barracuda, SpamCop etc) and they all have their own criteria to ascertain whether an email is acceptable or not.
However, deliverability is likely to suffer should you find yourself on one or more of them.
Are you on a blacklist?
If your email service provider doesn’t alert you to your IP being blacklisted then you have to do it manually. You can do this by using a blacklist’s website and searching your domain or IP address.