Problem:

You have a domain hosted with 3Essentials, and one sender who tries to send you email gets a bounce with the following message:

5.2.1 Mailbox unavailable. Your IP address XX.XX.XX.XX is blacklisted using ZSorbs-All-No-.6. Details: Dynamic IP Addresses See: http://www.sorbs.net/lookup.shtml?XX.XX.XX.XX

Resolution:

As one of the many protection mechanisms 3Essentials uses to protect it's customers from spam, we subscribe to a blacklist service from SORBS which identifies IP addresses that are dynamic vs static.  For instance, when you're connected to your residential ISP from your home, your PC (or home network) is dynamically assigned an IP address - this means the IP address may change (once a day, a week, a month... it varies).  Businesses will be assigned a static IP address, i.e., they will have the same IP address(es) throughout the life of their contract with their internet service provider.  Legitimate mail (SMTP) servers should be hosted on a static IP address, not a dynamic IP address (which might indicate a spammer who's set up a SMTP server for spamming from his home internet connection).  As such, it has become a widely used approach to not accept mail from servers which are using a dynamic IP address... and we do this by subscribing to the "dynamic IP blacklist service" from SORBS. 

On a rare occasion, SORBS will list an IP as a dynamic IP which is not dynamic.  As such, the SENDER who receive the bounce-back, should contact their ISP and provide them with the bounce-back message, including the URL for SORBS, and request they get the IP delisted.  It is standard practice for the ISP who owns the IP address to deal directly with the blacklisting service to address such issues.