Reputation can be very important in all walks of life, and email is no different. For senders with a good reputation, mail is more likely to be delivered to the inbox, giving your recipients more chances to read and interact. On the other hand, if your reputation is “not so hot,” you’ll be more likely to see your messages end up in the spam folder or even rejected altogether. So what exactly is this mystical “reputation,” and how is it determined?
IP Address Reputation
Before we address (no pun intended) the reputation aspect, let’s have a brief refresher on the nature of IP addresses. Every device connected to the internet – computer, router, even your smart thermostat – is assigned a numeric value known as the Internet Protocol address. This IP address is how your mail server is identified on the internet. When you send mail to a recipient, their mail provider will see where the message originated, identified by that IP address.
Most inbound mail servers will also attempt to use that IP address to determine whether the sender of the mail is trustworthy. If mail from that IP address is often sent to invalid addresses, or the mail generates too many complaints, the inbound server may choose to route incoming messages from that server to spam, or block them outright. Many mailbox providers also use third-party IP address blacklists like Spamhaus or Spamcop, choosing to filter or reject mail from IPs that appear on those lists.
A few years ago, IP address was the gold standard for sender reputation. As content filters that looked for “spam words” became less effective (MILLI0NS OF DOLLAR$ IN VI4GRA, anyone?), IP reputation helped receivers identify habitual bad senders. The receivers could then act on all mail sent from those servers instead of chasing down ever-morphing content to filter inbound mail. This often caused issues for ESPs with smaller clients, because many clients were often sending from the same IP address (This is one of the reasons many ESPs tend to be more strict with shared-IP clients – because mail you send can impact delivery for many other senders on your IP).
A fact that is not widely known outside tech circles is that the internet is running out of IP addresses. As a result, a new standard for IP addresses was created: IPv6. This standard allows for a seemingly infinite number of IP addresses – theoretically 340,282,366,920,938,463,463,374,607,431,768,211,456 to be exact. With that many IP addresses, anyone whose IP is blocked can simply start using a different IP address, essentially bypassing IP address-based filtering.
Domain Reputation
There are times that an IP address alone doesn’t give the whole picture. The shared IP pools used by some ESPs, as mentioned above, are a prime example. Another example might occur when multiple divisions of a single company or organization use the same IP addresses to send mail. Because of this, and the onset of the IPv6 standard, domain reputation has become increasingly important in mail delivery.
How domain reputation is checked can vary depending on the mail server. Some providers check only the domain the mail is actually sent from, while others look at all the domains in the message headers. Some providers, including Google, check every domain in the body of the message as well. So if you send your mail from example@example.com, you can bet the reputation of example.com will be checked, but other domains in your message could be checked as well. If you are using an ESP or other hosted mail solution, there may be domains in the header that belong to your host that will also be checked (another reason authentication is so important). And don’t forget those links in the body! Many filters will block a message if it contains links to domains that have been flagged as spam or dangerous. If you’re linking to a domain you don’t own, it’s a good idea to check its reputation using one of the handful of free online tools available (we’ve compiled a few on our Resources page).
Not surprisingly, a domain’s reputation is typically impacted by mail sent from that domain or using that domain in the body. However, it can also be affected by the reputation of that domain on the web as a whole. If a specific website has gotten a bad reputation for, say, gaming Google’s search engines or potentially scamming customers, that can also impact mail delivery.
The ‘Secret Sauce’ of Sender Reputation
When it comes to actually filtering inbound email, every mailbox provider uses a unique combination of factors to determine whether mail reaches the inbox. For many providers, IP address reputation still ranks at or near the top of the list of these factors. Domain reputation is typically very close behind, if not in the top spot. After that, there are typically various ingredients that make up the proprietary methods used by each organization. Trying to figure out the secret sauce for each ISP is a fruitless effort – even if you do manage to find a trick that bypasses or overcomes a specific filter for a time, it will invariably change and you’ll be left looking for another backdoor.
While the specific components of spam filters change constantly, your sender reputation – both IP and domain – will continue to play the largest role in getting your mail to the inbox. Manage that reputation well, and you’ll save yourself a lot of delivery headaches.
For more information on IP addresses or the IPv6 standard, check out these resources:
What is an IP Address? on HowStuffWorks
Word to the Wise – IPv6 archives
IPv6.com – A Beginner’s Look
– BG
Though most senders may be trying to pin down 
Return Path has long been a fixture in the email delivery community as a provider of tools for monitoring and improving inbox delivery rates, in addition to their newer data and intelligence products. One of Return Path’s most well-known offerings is their Return Path Certified program (formerly Sender Score Certification), which provides some additional metrics and benefits at certain ISPs for senders who meet the high standards of the program.