Last Updated on December 31, 2025 by Ewen Finser
The most frustrating part of my job is getting decent opens for months, and then one day it just… stops. There’s no dramatic change or especially spammy offers — it’s the same domains, same list source, and same cadence.
If you’ve also noticed that Outlook and Gmail are getting increasingly picky, you’re not just being paranoid. And when 1 in 6 emails never reach the inbox, it’s clear that action needs to be taken.
With that in mind, let’s break down the levers that reliably impact deliverability and how to improve Sender Score
What Is Sender Score?
Sender Score is a measure of how trustworthy your emails are, and it’s based on observed email behavior over time. It can be useful as a guiding measure, but it’s not a universal truth — and it’s definitely not what mailbox providers use as a single deciding factor.
But this doesn’t stop people from treating Sender Score like a KPI they can directly optimize. They chase quick bumps in a similar way to how they’d try to get quick boosts to CTR: tweak subject lines, change a few templates, refresh the copy, the list goes on.
In practice, it behaves more like a credit score. Complaints, bounces, engagement, and sending consistency accumulate, then the score moves when enough of those signals change in a sustained way. That’s also why it can lag behind quick fixes. If you clean up a list or slow down volume today, you may have improved the underlying risk profile, but the score will not reflect it immediately.
In reality, here’s what actually impacts Sender Score:
- Spam complaints and negative user feedback
- Unknown users, hard bounces, invalid addresses
- Engagement trends over time
- Sending patterns and volume consistency
- List quality and how it’s sourced
But what usually eats up time instead are surface tweaks: subject line experiments, template refreshes, warmup routines layered on top of the same unhealthy list. Those changes aren’t useless, but they just don’t mean a lot when the fundamentals are neglected.
Even if you adjust the right levers, it’s still important to keep expectations grounded. Different mailbox providers still make their own decisions, after all, so while you might have an improving Sender Score, you can still struggle in Gmail. Or you might look fine in one inbox while another is less forgiving.
That’s normal.
The Quickest Ways to Lose Sender Score
Most mysterious drops in Sender Score aren’t the direct result of some unknowable algorithmic update… they come from a handful of choices that feel practical in the moment but only stack the odds against you.
The first one is list sourcing. If you buy or scrape a list and tell yourself you’ll clean it later, you’re doing it in the wrong order. The damage doesn’t happen after you clean; it happens the first time you send a campaign, when you hit a pocket of dead addresses or people who never expected you. That pattern is exactly what reputation systems are designed to punish. You’re labeled as failing to maintain basic list hygiene — or worse, emailing addresses you didn’t earn. The math looks fine on paper, but the inbox providers read it as carelessness.
The second is volume. Spiky sending is one of the easiest ways to look out of control: a long break, then a big blast, then another break. Even if your offer is great, that cadence smells like low-quality sender behavior, and it tends to pull your reputation down fast.
Then there’s the quintessential spam lever. When someone reports your email as spam, mailbox providers are getting a clean, explicit signal that your messaging is unwanted. These complaints are brutal because they’re not interpretive; opens and clicks can be messy, but complaints are a clear signal of someone raising their hand and saying “stop.” Enough of that, and you start to look both irrelevant and risky.
The last one is mixing worlds. Cold outreach behavior (lower engagement, higher complaints, more ignoring) doesn’t belong on the same domain as your opt-in newsletter or lifecycle emails. You can sometimes get away with it, but as your audience broadens and the top of the funnel gets even wider, continuing to blast is how a small problem becomes big.
Warning signs you should watch out for:
- Hard bounce rates increasing week over week
- Spam complaints beyond the rare one or two
- Placement issues in Gmail/Outlook without a big change
- Engagement dropping with the same volume of emails
The Foundational Fixes
If your reputation ever “mysteriously” improves, it’s almost always because you’ve taken the time to improve the foundational basics.
List Hygiene That Isn’t Fake “Cleaning”
Proper list hygiene starts before you send, not after. Deleting hard bounces from a report and calling it “cleaning” is like mopping the floor after the pipe bursts and leaving the leak untouched.
In practice, proper hygiene usually means:
- Suppressing chronic non-engagers
- Removing invalid addresses as soon as they surface
- Letting truly inactive segments go
The hard part is actually psychological. Dead segments might feel like potential, but mailbox providers just see a long tail of people who never open, click, or reply. To them, it’s a sign that your mail isn’t all that important.
Trying to resurrect inactive segments is almost never worth the risk. If someone hasn’t engaged over multiple campaigns, sending more mail to prove relevance usually does the opposite.
Hygiene is, more than anything, being honest with your data.
Consistency Beats Intensity
If there’s one lever that most stabilizes sender reputation, it’s consistency.
Mailbox providers pay close attention to volume patterns — a steady cadence (even a modest one) is easier to trust than long silence followed by a surge. Spikes create suspicion, especially on new lists with new content or new infrastructure.
This is also where people accidentally sabotage themselves by changing too much at once. New copy, new domain, new schedule… then two weeks later, asking exactly why things changed. When five variables move together, reputation systems can’t tell what’s improved.
When volume changes are necessary, make sure to ramp slowly. When something improves, let it run long enough to be recognized as a pattern. This is the unglamorous reason that Sender Score often lags behind good decisions: the system needs to see that you’ve actually changed how you send.
This is also the point where visibility starts. When you’re dialing in cadence and volume, knowing where messages are landing makes it much easier to avoid overcorrecting. That’s when inbox placement tools like InboxAlly become one of the most useful tools you can have.
Authentication Basics
Authentication helps you remove doubt.
- SPF defines which servers are allowed to send on your behalf.
- DKIM signs your messages so they can’t be altered in transit.
- DMARC checks whether both of these align with the domain recipients see on their end.
What really matters is alignment. When the domain in your From address matches what SPF and DKIM authenticate, you look coherent. When it doesn’t, you look sloppy, or even deceptive.
What authentication does not do is guarantee inbox placement. Plenty of fully authenticated senders land in spam every day. But without it, you’re asking mailbox providers to trust you while giving them reasons not to.
Authentication won’t exactly win you the game, but it will keep you from getting disqualified before you even start.
Engagement: The Lever Most People Misunderstand
After you’ve fixed the obvious stuff like the list, sending volume, and basic authentication, Sender Score becomes quite predictable.
But this is where a lot of people get stuck anyway, because “engagement” sounds fluffy — like something you either have or you don’t.
The mistake most people make is thinking that engagement is strictly a copywriting problem. Sometimes it is, but most of the time, it’s more of an audience problem than anything else. You can write the best email but still get low engagement if you’re sending it to people who don’t care, don’t remember you, or never asked for it in the first place.
What Good Engagement Looks Like When You’re Starting From Zero
Early on, it’s tempting to treat your list as one big blob. That’s usually when engagement looks confusing: some people love you, most people do nothing, and a small percentage quietly hate you (at least enough to complain).
A better approach is to accept that engagement isn’t evenly distributed. There’s almost always a smaller group that’s genuinely interested: recent signups, recent purchasers, people who clicked something in the last 30–60 days, and people who’ve replied before. That group is the foundation — if you’re rebuilding your reputation, they’re also the safest audience to keep sending to while you stabilize.
The moment you start paying attention to who is engaging and not just how much engagement you’re getting, the decisions get easier. You stop asking, “How do we raise our open rate?” and start asking, “Why are we sending to people who haven’t reacted to us in months?”
The Practical Stuff That Lifts Engagement
Consistent engagement isn’t just a conversion driver — it also buys you room to send without everything breaking the moment you make a change.
But when engagement is low, the fix usually isn’t “be more clever.” It’s refining the match between message and recipient, and then making the experience frictionless.
A few things tend to move the needle here:
- Segment by recency, not just by persona
- Send less to colder segments
- Keep subject and body aligned (no bait-and-switches)
- Protect your best audience from experiments and spikes
- Make it easy to leave (one-click unsubscribes)
This last point is especially worth dwelling on. People often fight unsubscribes like they’re losses… but unsubscribes are actually a positive. If someone doesn’t want to hear from you, it’s best that they leave.
This is also where measurement can mess with your head. Open rate is less reliable than it used to be, and it varies by provider and setup. That doesn’t mean engagement doesn’t matter; it means that you should look at it as a pattern between various other metrics and not a single magic number. Replies, clicks, complaint rate, bounce rate, and inbox placement trends together will tell you a lot more truth than obsessing over a one-point change in open rate.
When it comes time to test changes in engagement, it’s good to have an idea of where these messages are landing. I often use InboxAlly here as a simple sanity check while rebuilding trust, mostly to avoid overcorrecting based on gut feel alone.
A Simple Recovery Plan When Your Score Has Already Tanked
If your Sender Score has already dropped, don’t panic! Stop doing whatever started the decline, and then rebuild trust in a way that providers can observe.
- Pause broad sends and stop the bleeding: Cut anything wide (full-list newsletters, big promos, large outbound batches), and keep only the essentials (password resets, receipts, critical lifecycle). The point is to stop generating new complaints and bounces while you figure out what changed.
- Isolate the damage source: Pick one likely culprit at a time. Is it a new list, a new domain, a new sending tool? If everything changed all at once, roll back the riskiest variable first (usually volume or audience expansion).
- Shrink to your most engaged segment: Send only to people who have shown recent interest. Look for recency and intent: opened/clicked/replied recently, signed up recently, purchased recently, or actively used the product. Everyone else gets suppressed for now. This usually reduces unknown users and complaints immediately, which buys you time to fix things.
- Ramp back up with steady volume and tight monitoring: Increase in controlled steps. Keep timing consistent, and watch hard bounces/unknown users, complaint rate, and inbox placement changes by provider. Improvements often need some time, so the job is to hold the new pattern long enough that it becomes the new normal.
While you’re in recovery mode, knowing what’s actually delivered is not enough; you need to know where messages are landing while you make changes. That’s where you can turn to InboxAlly to make sense of what’s going on.
What to Track
Sender Score is useful, but it’s not the thing you should obsess over. If you only watch the score, you usually react late because it tends to move after the real problems have already shown up elsewhere.
The first time your deliverability gets in trouble, it’s tempting to stare at the open rate until something “feels” better. But open rate has lost much of its meaning since Apple introduced Mail Privacy Protection, which masks actual user behavior.
What you want instead is a small set of indicators that map directly to sender trust, which most mailbox providers expose parts of through postmaster-style dashboards and feedback loops:
- Hard bounce rate and unknown users
- Spam complaint rates (not unsubscribes)
- Engagement trends (not one-off spikes)
- Inbox placements by provider, if measurable
- Volume consistency week to week
- Domain and IP reputation over time
Watching these inputs lets you adjust early while changes are still small and reversible, so think ahead!
Improving Sender Score with the Right Mindset and Tools
Hopefully, this gives you a clearer picture of how to approach sender reputation going forward.
Email is rarely simple (deliverability even less so), so focus on the things you can actually control. Send thoughtfully, adjust carefully, and give changes enough time to register.
And if at any point it feels confusing, or if you just want to sanity-check what you’re seeing, give a deliverability tool like InboxAlly a try. It helps you identify and adjust the levers that actually make a difference and evolve beyond guesses and gut feelings.