Your IP represents the sending environment, typically a shared IP owned by your email service provider (ESP). Knowing the sending IP reputation gives insight into the reputation of your ESP, which directly influences connection-level blocks and sending delays. In other words, a poor IP reputation means more bounces and slower delivery.
When it comes to shared IPs, if an ESP accepts customers who are prone to spam complaints and excessive bounces, receivers will lower their opinion of the traffic coming from that IP. At the same time, being on a shared IP with many high-quality customers typically improves deliverability by buffering effects when an inevitable sending mistake happens. The performance of a shared IP pool is directly related to the ESP’s standard for their customers, so it’s a good sign when creating your account involves completing questionnaires and replying to support messages.
So how can you tell if your ESP is following best practices for your deliverability? First you need to see what IP your messages are sent over. Open any message sent from your ESP (not triggered through an ESP’s testing feature, since tests are sometimes sent over different IPs than live sends), and view the message headers. What those headers look like may vary based on who’s being sent to, but I find the simplest way to find the sending IP (especially in Gmail) is to look at the SPF authentication results:
In the raw, unformatted headers, you may see something like this:
Authentication-Results: spf=pass (sender IP is 220.127.116.11)
Received-SPF: pass (domain of example.com designates 18.104.22.168 as permitted sender)
For those sending over a shared pool (multiple IPs), open as many live messages as you can that were sent in the past week. Going back too far could yield inaccurate results, since some ESPs move customers to different IP pools periodically. Make note of the various IP addresses you see in those headers, and you’ll likely even notice a pattern that they’re all in the same range. For example, if I see that I’m sending over 22.214.171.124 and 126.96.36.199, I might get additional insight from checking the reputations of 50.31.156.[126-127].
If you use custom SPF/DKIM authentication and have signed up for Google Postmaster Tools (you should!), refer to the “IP reputation” visualization there to get a shortcut to both finding your sending IPs as well as understanding their reputation.
Of course there are also public IP check tools you can use to understand its reputation for multiple receivers. The most popular likely being ReturnPath’s Senderscore service, and you might need to create a free account there to see enough data. A SenderScore in the 90s is ideal, and anything lower (especially below the 80s) you’ll want to reach out to your ESP to investigate.
Another public service I like to use is Talos Intelligence by Cisco, looking for a grade there called “Email Reputation”. You may also get some insight from ReputationAuthority, although it tends to give a “neutral” reputation to most IPs (especially those where they’ve received little data). For both of these tools, it’s less informative to see a “good” or “neutral” grade which they tend to hand out to most IPs. Instead the most insight comes from whether the IP is less than “neutral”, meaning it definitely needs the attention of a deliverability professional or your ESP.
Beyond reputation, you may also want to know if the IP is currently on any blacklists. It’s good to note that these may not have any direct effect on reputation, simply because a receiver has to actually reference that specific blacklist for it to make any difference in deliverability. There are countless public blacklists because pretty much anyone with a computer can make one, so it helps to do some research first (or talk to your deliverability team) to know whether the blacklist is widely used and reputable. The quickest blacklist check for an IP is probably MultiRBL, but if you see a listing there, be sure to follow the link to blacklist’s website to check again directly (sometimes MultiRBL’s cache is inaccurate or delayed in marking de-listings).
Now that you’ve collected some data on your sending IP, what’s next? If you’ve found your IP to be in good standing everywhere, you’re working with a top-notch sending environment that will allow your own good sending habits to shine! Of course continue to re-check your IP reputation regularly in case there are any changes.
If you’ve found the IP to have some reputation issues, definitely reach out to your ESP to determine the work to be done to correct it. For dedicated IPs, they should be able to advise you on improving sending practices and help with reaching out to affected ISPs and blacklists. For shared IPs, they’ll need to launch their own investigation into any customers that may have caused the reputation drop. A responsive ESP to these kinds of issues is a clue whether this is a one-off situation or a common reputation problem that may require an email provider switch.
[Originally posted here.]