You can write the best email in the world, but if it lands in the spam folder, it might as well not exist. Email deliverability is the foundation every other part of your email marketing program is built on, and it is one of the most misunderstood aspects of email marketing for e-commerce brands.
The good news is that poor email deliverability is almost always fixable. The bad news is that it is much easier to damage your deliverability than it is to repair it, which means the best strategy is to understand it before you have a problem rather than after.
This article covers what email deliverability actually is, what causes it to break down, and what you can do in Klaviyo to protect and improve it.
What Email Deliverability Actually Means
Email deliverability refers to the likelihood that an email you send will arrive in your subscriber’s inbox rather than their spam folder or get blocked entirely before it reaches them. It is not a binary pass or fail: your emails might be reaching 95 percent of subscribers’ inboxes, or 60 percent, or somewhere in between.
Inbox providers like Gmail, Yahoo, and Outlook use complex filtering systems to decide where your emails land. Those systems look at a combination of technical authentication, your sending reputation, your engagement history, and the content of your emails. All of those factors interact, and all of them are within your control to some degree.
Technical Authentication: The Non-Negotiables
Before anything else, your sending domain needs to be properly authenticated. This is the technical foundation of deliverability, and without it, inbox providers have no way to verify that your emails are actually coming from you.
There are three authentication protocols you need to have in place: SPF, DKIM, and DMARC.
SPF (Sender Policy Framework) tells inbox providers which servers are authorized to send email on behalf of your domain. Setting up SPF for Klaviyo means adding a TXT record to your DNS that includes Klaviyo’s sending infrastructure.
DKIM (DomainKeys Identified Mail) adds a digital signature to your emails that verifies the message has not been tampered with in transit. Klaviyo generates DKIM keys for you and provides the DNS records you need to add to your domain settings.
DMARC (Domain-based Message Authentication, Reporting, and Conformance) tells inbox providers what to do if an email fails SPF or DKIM checks, and gives you reporting on authentication failures. A basic DMARC policy set to “p=none” (monitor mode) is a good starting point, with the option to move to “p=quarantine” or “p=reject” as your program matures.
All three of these are configured through your domain provider’s DNS settings, and Klaviyo provides step-by-step instructions for each one in the Email Sending Domains section of your account settings. If you have not set these up yet, this is the single most important thing you can do for your deliverability.
Sending Reputation
Your sending reputation is a score that inbox providers assign to your domain and IP address based on how recipients respond to your emails. It is not a number you can directly see, but it directly determines whether your emails reach the inbox or get filtered.
The primary signals that build or damage your sending reputation are your spam complaint rate, your unsubscribe rate, your open and click rates, the number of invalid email addresses on your list (bounces), and your list growth practices.
A high spam complaint rate is the fastest way to damage your reputation. Gmail and Yahoo have made it clear that a complaint rate above 0.1 percent will result in filtering, and anything above 0.3 percent risks a full block. Monitor your spam complaint rate in Klaviyo regularly and treat any spike as an urgent signal to investigate.
A soft bounce is a temporary delivery failure. This happens when the email address is valid but the message could not be delivered at that moment, typically because the recipient’s inbox is full, the receiving server was temporarily unavailable, or the message was flagged as too large. A single soft bounce is not cause for alarm, but repeated soft bounces to the same address across multiple sends are a signal that something is persistently wrong, whether that is an abandoned inbox, a server that consistently rejects your emails, or an email address that is no longer in active use. Klaviyo tracks soft bounces automatically and will suppress a contact after a defined number of consecutive soft bounces, protecting your sending reputation without requiring you to manually monitor each case.
Hard bounces, which happen when an email address does not exist or is permanently unreachable, damage your reputation because they signal to inbox providers that your list is poorly maintained. Klaviyo automatically suppresses hard bounces, but if you are importing contacts from outside sources, clean your list before importing to avoid adding invalid addresses.
List Health and Its Impact on Deliverability
The quality of your list matters more than the size of it. A large email list full of unengaged or invalid contacts is a deliverability liability. A smaller list of engaged, opted-in subscribers will reach the inbox far more reliably.
List hygiene means regularly suppressing unengaged contacts who have not opened or clicked in six months or more. In Klaviyo, you can build a segment of unengaged subscribers and either run a re-engagement campaign before suppressing them, or suppress them directly if the window of inactivity is long enough.
One of the hardest mindset shifts in email marketing is learning to let go of contacts. It feels counterintuitive to suppress people from a list you worked hard to build, but holding on to unengaged contacts is one of the most common ways brands quietly damage their own deliverability. When you email your entire list regardless of engagement, you are sending a large portion of your sends to people who are not opening, not clicking, and potentially marking you as spam. Inbox providers read that pattern as evidence that your emails are not wanted, and they adjust where your messages land accordingly. A list of 10,000 engaged subscribers who regularly open and click will consistently outperform a list of 50,000 where 35,000 have not engaged in six months. The size of your list is not the goal. The health of your list is. Suppressing unengaged contacts is not giving up on them, it is protecting your ability to reach the people who do want to hear from you.
List Consent
Never purchase an email list. Ever. Purchased lists contain invalid addresses, spam traps (addresses set up specifically to catch senders using purchased lists), and people who have no relationship with your brand. Sending to a purchased list will damage your reputation immediately and significantly.
Make sure every contact on your list has explicitly opted in to receive marketing emails from you. Importing customers who gave you their email for a purchase but did not opt in to marketing is a common mistake that leads to high spam complaint rates, because those customers never expected to hear from you.
Content That Triggers Spam Filters
Beyond authentication and list health, the content of your emails can also affect deliverability. Spam filters analyze the text, images, and HTML of your emails looking for patterns that match known spam signals.
Certain words and phrases have historically been associated with spam: “Free!!!,” “Act now,” “Limited time offer,” and similar high-pressure language are commonly flagged. This does not mean you can never use urgency or promotional language, but it does mean being thoughtful about how you use it and avoiding an overconcentration of spam-associated terms in a single email.
The ratio of text to images in your email also matters. Emails that are predominantly images with very little text are a red flag for spam filters, because spammers often try to hide their message content inside images to avoid text-based filtering. A healthy balance of text and images, with proper alt text on every image, is the safest approach.
Sending emails with broken HTML or excessive tracking links can also trigger filters. In Klaviyo, using the built-in template editor rather than custom HTML minimizes the risk of HTML issues, and Klaviyo’s built-in link tracking is designed to work cleanly with inbox providers.
Warming Up a New Sending Domain
If you are setting up a new domain or migrating from another platform, you cannot jump straight to sending your full volume from day one. Inbox providers have no sending history for your new domain, and a sudden flood of emails from an unknown domain is a classic spam signal.
Warming up a new domain means starting with a small volume of sends to your most engaged subscribers and gradually increasing over two to three weeks. Start by sending a few hundred emails in the first week, then scale to a few thousand, then to your full list volume. Keep your engagement rates strong throughout the warm-up period by targeting only your highest-engagement contacts until your reputation is established.
Monitoring Deliverability in Klaviyo
Klaviyo shows you key deliverability signals in your account. Pay attention to your deliverability rate (the percentage of emails sent that were successfully delivered), your spam complaint rate, and your bounce rate.
Google and Yahoo also offer free inbox provider tools, Google Postmaster Tools and Yahoo Sender Hub, that show you your sending reputation directly from those providers’ perspective. Connecting your sending domain to these tools gives you visibility into how the two largest inbox providers see you, which is valuable information for any brand sending at meaningful volume.
Deliverability is not something you set up once and never think about again. It is an ongoing practice of keeping your list clean, sending relevant content, monitoring your metrics, and addressing any signals of trouble before they compound into a bigger problem.
If you want help auditing your current deliverability setup in Klaviyo or resolving a deliverability issue, we are experienced in exactly this area and happy to help.
Continue reading: How often should you send marketing emails? A data-backed answer and Understanding your Klaviyo dashboard: The metrics that actually matter.