The sending reputation (or "level of trust") of an IP address greatly influences your messages' deliverability in that it directly influences connection-level blocks and sending delays. In other words, a good IP reputation means less bounces and faster delivery.That's because the IP represents your sending environment, and knowledge about other senders and messages using that same sending environment allows receivers to make upfront assumptions about the quality of your mail too. So how can you tell if your email provider is following best practices for your deliverability?
First, the performance of a shared IP pool is directly related to the email service provider’s (ESP) standard for their customers, so it’s a good sign when creating your account involves completing questionnaires and replying to support messages. If they accept customers prone to spam complaints and excessive bounces, receivers will lower their opinion of the traffic coming from that provider's IPs. 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.
Next you'll need to find out over which IPs your messages are sent. 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.
If you're not using Google Postmaster Tools or there's no data available there, then open any message sent from your ESP (not triggered through any "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 the inbox provider, but the simplest way to locate the sending IP (especially in Gmail) is to look at the SPF authentication results:
In the raw, unformatted headers, you should see something like this:
Authentication-Results: spf=pass (sender IP is 22.214.171.124)
Received-SPF: pass (domain of example.com designates 126.96.36.199 as permitted sender)
For those sending over a shared pool (multiple IPs), open as many sent messages as you can from the past week. Looking at messages sent more than a couple of weeks ago 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 188.8.131.52 and 184.108.40.206, I'll get additional insight by checking the reputations of 50.31.156.[116-121].
Now that you know your sending IPs, there are public IP check tools you can use to understand its reputation for multiple receivers. The most popular is likely ReturnPath’s Senderscore service. You just need to create a free account there to see enough data. Any SenderScore in the 90s is excellent, but anything lower (especially below the 80s) means you’ll want to reach out to your ESP as soon as possible to investigate and resolve.
Another public lookup tool for IP reputation is Talos Intelligence by Cisco, looking there for the “Email Reputation” grade. Comcast is known for referencing BrightCloud's IP list, which tells whether there's a perceived "threat" risk for your IP. You may also get some insight from ReputationAuthority, although it tends to give a “neutral” reputation to most IPs (especially those with very 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” ("bad" or "poor"), meaning this IP definitely needs the attention of a deliverability professional or your ESP, stat!
Beyond reputation, you may also want to know if the IP is currently on any notable anti-spam or block lists (sometimes called "blacklists"). The quickest check for an IP is probably MultiRBL, and if you see a listing there, be sure to follow the link to list's website to check again directly (sometimes MultiRBL’s cache is inaccurate or delayed in marking de-listings).
Of course many of these public block lists may not have an effect on reputation if who you're sending to isn't actually referencing that specific list. There are countless block lists in existence because anyone with a computer can make one, so it helps to do some research first (or talk to your deliverability team) if you're curious whether a specific list is widely used and reputable.
Now that you’ve collected some data on your sending IP, what’s next? 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. 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.
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! Continue to re-check your IP reputation regularly (once a month, or as concerns arise) in case there have been any changes.