Sunday, June 7, 2015

What is a DNS Blacklist? Mail Server?

What is a Mail Server?

Trust me. 

What is a Mail Server?

With the click of a mouse button, you can send an email from one point of the globe to another in a matter of seconds. Most of us take this process for granted, giving little thought to how it actually works. It's easy to understand how standard snail-mail gets from point A to point B - but how does an email message make its way from a sender to a recipient? The answer to that question revolves around something called a mail server. You can learn more about the role that mail serves play in email delivery by reading on below.

What is a Mail Server?

A mail server is the computerized equivalent of your friendly neighborhood mailman. Every email that is sent passes through a series of mail servers along its way to its intended recipient. Although it may seem like a message is sent instantly - zipping from one PC to another in the blink of an eye - the reality is that a complex series of transfers takes place. Without this series of mail servers, you would only be able to send emails to people whose email address domains matched your own - i.e., you could only send messages from one account to another account.

Types of Mail Servers

Mail servers can be broken down into two main categories: outgoing mail servers and incoming mail servers. Outgoing mail servers are known as SMTP, or Simple Mail Transfer Protocol, servers. Incoming mail servers come in two main varieties. POP3, or Post Office Protocol, version 3, servers are best known for storing sent and received messages on PCs' local hard drives. IMAP, or Internet Message Access Protocol, servers always store copies of messages on servers. Most POP3 servers can store messages on servers, too, which is a lot more convenient.

The Process of Sending an Email

Now that you know the basics about incoming and outgoing mail servers, it will be easier to understand the role that they play in the emailing process. The basic steps of this process are outlined below for your convenience.

Step #1: After composing a message and hitting send, your email client - whether it's Outlook Express or Gmail - connects to your domain's SMTP server. This server can be named many things; a standard example would be

Step #2: Your email client communicates with the SMTP server, giving it your email address, the recipient's email address, the message body and any attachments.

Step #3: The SMTP server processes the recipient's email address - especially its domain. If the domain name is the same as the sender's, the message is routed directly over to the domain's POP3 or IMAP server - no routing between servers is needed. If the domain is different, though, the SMTP server will have to communicate with the other domain's server.

Step #4: In order to find the recipient's server, the sender's SMTP server has to communicate with the DNS, or Domain Name Server. The DNS takes the recipient's email domain name and translates it into an IP address. The sender's SMTP server cannot route an email properly with a domain name alone; an IP address is a unique number that is assigned to every computer that is connected to the Internet. By knowing this information, an outgoing mail server can perform its work more efficiently.

Step #5: Now that the SMTP server has the recipient's IP address, it can connect to its SMTP server. This isn't usually done directly, though; instead, the message is routed along a series of unrelated SMTP servers until it arrives at its destination.

Step #6: The recipient's SMTP server scans the incoming message. If it recognizes the domain and the user name, it forwards the message along to the domain's POP3 or IMAP server. From there, it is placed in a sendmail queue until the recipient's email client allows it to be downloaded. At that point, the message can be read by the recipient.

How Email Clients are Handled

Many people use web-based email clients, like Yahoo Mail and Gmail. Those who require a lot more space - especially businesses - often have to invest in their own servers. That means that they also have to have a way of receiving and transmitting emails, which means that they need to set up their own mail servers. To that end, programs like Postfix and Microsoft Exchange are two of the most popular options. Such programs facilitate the preceding process behind the scenes. Those who send and receive messages across those mail servers, of course, generally only see the "send" and "receive" parts of the process.

At the end of the day, a mail server is a computer that helps move files along to their intended destinations. In this case, of course, those files are email messages. As easy as they are to take for granted, it's smart to have a basic grasp of how mail servers work.

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What is a DNSBL?

Domain Name System Blacklists, also known as DNSBL's or DNS Blacklists, are spam blocking lists that allow a website administrator to block messages from specific systems that have a history of sending spam. As their name implies, the lists are based on the Internet's Domain Name System, which converts complicated, numerical IP address such as into domain names like, making the lists much easier to read, use, and search. If the maintainer of a DNS Blacklist has in the past received spam of any kind from a specific domain name, that server would be "blacklisted" and all messages sent from it would be either flagged or rejected from all sites that use that specific list.

DNS Blacklists have a rather long history in web terms, with the first one being created in 1997. Called the RBL, its purpose was to block spam email and to educate Internet service providers and other websites about spam and its related problems. Although modern DNS Blacklists are rarely used as educational tools, their function as an email blocker and filter still serves as their primary purpose to this day. In fact, almost all of today's email servers support at least one DNSBL in order to reduce the amount of junk mail clients using their service receive. The three basic components that make up a DNS Blacklist - a domain name to host it under, a server to host that domain, and a list of addresses to publish to the list - also haven't changed from the time when the RBL was first created to today.

Since then, dozens of different DNSBL's have sprung up and are available for use, and they all have their own lists that are populated based on what does or doesn't meet their own standards and criteria for what a spammer is. Because of this, DNS Blacklists can vary greatly from one to the other. Some are stricter than others, some only list sites for a set amount of time from the date the last piece of spam was received by the maintainer versus others that are manually maintained, and still others not only block IP addresses, but also entire ISP's known to harbor spammers. This results in some lists working better than others because they are maintained by services with a greater level of trustworthiness and credibility than competing lists might have. Users can also use these differences to decide on which DNS Blacklist works best for them depending on what their specific security needs are. Less lenient lists might allow more spam to get through, but might not block non-spam messages that have been misidentified on lists that have stricter guidelines for what goes on or what is left off of it. To help facilitate this, DNS Blacklists that are intended for use by the public will usually have a specific, published policy detailing what a listing means and must adhere to the criteria laid out in it in order to not only attain public confidence in their services, but to sustain it as well.

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How to Remove an IP Address from a Blacklist

(Go to our Blacklist Check page to find out if your IP address is listed on an anti-spam database. This article explains why that happens and how to get off a blacklist.)

Each blacklist database has its own criteria for flagging IP addresses and compiling its own list of online offenders. Those criteria could include a variety of "listings": technical, policy, and evidence-based.

  • Technical listings occur mostly from mail-server configuration issues, such as missing or incorrect reverse DNS records, missing or incorrect banner greetings, and mail servers operating within a suspicious range of IP addresses.
  • Policy listings are based on an operator that does not wish to receive email from certain countries, or ISPs, that have a history of not honoring "unsubscribe" requests.
  • Evidence-based listings are those where the operator has received direct (or indirect) evidence that an IP address has been involved in sending unsolicited emails.

If your IP address has been blacklisted and you want to investigate, you'll need to visit the blacklist's website and do a lookup on your IP address. Most blacklist databases will provide general listing reasons, but don't list specific email addresses tied to blacklisted IP addresses.

Getting "unblacklisted."

If you're able to find out why you were blacklisted, you can try to get it reversed. (You may want to work with someone who is technically savvy to better help you.)

To start with, take time to ensure your network and mail server are configured correctly and all the details are in order for resolving the issues, as prescribed by the blacklist. For example, they may ask you to correct both forward and reverse DNS records, as well as SMTP banners. In addition, you can do the following:

  • Scan all computers on your network for viruses
  • See if there are any known and needed "patches" (updates and fixes) for your operating system
  • Configure routers more securely
  • Establish and enforce stronger passwords

Following the blacklist-removal process.

You want to be removed from any blacklists because databases often share IP addresses that have been listed. If you think you've fixed things on your end, go back to the blacklist's site and follow their instructions for the IP address removal process. Here's what you're likely to come across:

  • Self-Service Removal. There are a few blacklists with a self-service removal feature that lets you take your IP address off the list without much trouble. However, you'll want to make sure you've resolved any issues before doing this. If you don't and your IP address gets listed again, it won't be easy to get it removed that next time.
  • Time-Based Removal. Most blacklists have a built-in, automatic process that removes lower-level listings (IP addresses that are light offenders) within a week or two. But if the IP address had sent spam more than once or did a high volume, the time period will be longer.

Be nice...and see what happens.

When you're trying to get off a blacklist, you'll get farther along if you follow the rules and cooperate. If you are truly innocent of any deliberate wrongdoing (or if you made an honest mistake), let them know. The more open and direct you are with a listing database, the simpler it may be to have your IP address taken off the blacklist.

Keep this in mind:

  1. Their priority is to reduce the spam on their email platform for their customers—their goal isn't to prevent you from sending emails.
  2. Spam is a serious problem. They don't blacklist lightly. It's their way of trying to identify and prevent real problems.
  3. Blacklists are legal because they are designed to prevent fraud or other activity that disrupts normal business. We all need to accept that fact.
  4. If you made a mistake and were blacklisted, don't make the same mistake again. You likely won't be forgiven a second time.

You might be able to resolve any blacklist issues online. If not, and the blacklisting is troublesome for you, consider contacting the list by phone and try to resolve the issue that way.

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