Your email isn’t broken, your reputation is
You hit send. The email looks fine in your own test inbox. Then you find out half your list never saw it, because it quietly slid into spam or, worse, got blocked before it arrived.
This is almost never about one bad subject line. Email deliverability is a reputation game, and mailbox providers like Gmail and Outlook have got very good at deciding whether you deserve the inbox.
Here is what they are actually judging, and what you can do about it.
What mailbox providers are really checking
Three things carry most of the weight.
Authentication. Can the provider prove the email genuinely came from you, and not someone spoofing your domain? If it can’t, you start on the back foot.
Sending reputation. Every sending domain and IP builds a track record. Send clean, wanted mail consistently and your reputation climbs. Send to dead addresses, collect spam complaints, or suddenly blast ten times your usual volume, and it drops.
How people react. This is the one people underrate. Opens matter less than you think. What really counts: do people reply, star you, move you to a folder, or do they delete without reading, ignore you, or hit “report spam”? Gmail watches this closely. A handful of complaints per thousand emails is enough to hurt you.
You can’t talk your way past this. But you can build the foundations properly, and most stores haven’t.
The technical bit, in plain English
There are three records that live in your domain’s DNS settings. You set them once, check them occasionally, and they do a lot of quiet work.
SPF is a list of the servers allowed to send email for your domain. When Klaviyo or your outreach tool sends on your behalf, the receiving server checks that list. If the sender isn’t on it, that’s a red flag.
DKIM adds an invisible signature to every email, a bit like a wax seal. The receiving server checks the seal is intact, which proves the message wasn’t tampered with and really came from your domain.
DMARC ties the two together and tells providers what to do when a message fails the checks: ignore it, send it to spam, or reject it. It also sends you reports on who is using your domain to send mail, which is how you spot spoofing.
If you take one thing from this post: get all three set up correctly. Gmail and Yahoo now effectively require them for anyone sending in volume. Missing DMARC in 2026 is like turning up to a meeting with no ID.
Your email platform will have a guide for adding these. It’s a fiddly ten minutes in your DNS, not a rebuild.
Warm-up and volume: don’t sprint from a standstill
A brand new sending domain has no reputation, good or bad. If your first ever campaign goes to 40,000 people, that looks exactly like what a spammer does. Providers get suspicious and throttle you.
The fix is warm-up. Start small, send to your most engaged people first, and build volume over a few weeks. The same goes for a fresh inbox used for cold outreach: a new mailbox firing off fifty emails on day one is asking for trouble. Ramp gently.
Then keep volumes steady. Big unpredictable spikes read as risk.
List hygiene: the unglamorous fix that works
Sending to addresses that bounce, or to people who stopped caring two years ago, is the fastest way to wreck your reputation.
- Remove hard bounces automatically. Your platform can do this for you.
- Set up a sunset flow: if someone hasn’t opened anything in, say, 90 to 120 days, stop mailing them or drop them to a low-frequency segment.
- Never buy a list. Bought lists are full of spam traps that get you blocklisted.
- Use double opt-in on signups if complaints are creeping up.
A smaller list of people who want to hear from you will always beat a big list of ghosts. Better for deliverability, better for revenue.
Content that trips the filters
The words matter less than they used to, but some habits still hurt:
- A single giant image with barely any real text.
- Link shorteners, or links that don’t match where they claim to go.
- Shouting: all caps, rows of exclamation marks, “FREE!!!”, “ACT NOW”.
- Attachments in cold outreach.
- A broken or hidden unsubscribe link. Make it one click. Burying it just causes complaints, and complaints are what actually sink you.
Write like a person. Plain, useful, easy to leave. That is what the filters have been trained to reward.
Why this matters on both sides of your email
For Klaviyo ecommerce email, deliverability is money. Your flows, abandoned basket, browse abandonment, post-purchase, only earn if they arrive. A store with a poor sender reputation is quietly losing sales it will never see in a report.
For B2B outreach, it is more brutal still. One cold campaign to a bad list from an unwarmed inbox can burn your main domain, and suddenly your normal business email starts landing in spam too. We usually recommend running outreach from a separate domain for exactly this reason, so a bad week never touches the address you rely on.
Where to start
If you do nothing else: confirm SPF, DKIM and DMARC are in place, stop mailing dead addresses, and make unsubscribing easy. That covers most of the problem for most stores.
Deliverability isn’t scary once the foundations are right. It is mostly discipline, and it pays you back quietly for years.
If you’d like a hand auditing your setup or fixing it properly, we’re happy to help. Our day rate starts from £350 for longer projects, every job gets a fixed quote first, and we reply within the hour, 6am to midnight. Get in touch.