Outlook Doesn't Just Check Your Domain: It Checks Your IP. Why Shared Sending Infrastructure Is Costing Klaviyo Brands Microsoft Inbox Placement

If your SPF, DKIM and DMARC all pass, your domain reputation looks fine, and your Klaviyo emails going to spam is a problem mainly at Outlook, Hotmail and Live addresses, the sending IP is a likely cause. In our experience, Microsoft weighs IP reputation heavily alongside domain reputation, and most Klaviyo brands send from shared IPs, so other senders' behavior can affect how Outlook treats you.

Key takeaways

• Gmail leans mainly on domain reputation. In our experience, Microsoft gives IP reputation a lot of weight too.

• Most Klaviyo accounts send from shared IP pools, which means a shared reputation.

• A Microsoft-only spam problem with clean authentication points toward the IP rather than your domain.

• You can't fix a shared IP's reputation yourself, but you can limit the damage and, at enough volume, move to dedicated sending.

You've checked your authentication and it passes. Every domain tool says your reputation is healthy. Yet Klaviyo emails going to spam keeps showing up at Outlook, Hotmail and Live, while Gmail seems fine. That pattern usually means you're looking at the wrong layer.

Microsoft weighs something Gmail weighs less

Gmail's filtering leans heavily on domain reputation. If your domain is authenticated and your sending looks clean, Gmail tends to treat you consistently, even on a shared IP with other Klaviyo customers.

In our experience, Microsoft's filtering also puts significant weight on the IP address the mail comes from. So the IP, not just the domain in the From line, plays a big part in whether a message reaches the Outlook inbox or the Junk folder. That explains a pattern many Klaviyo brands hit: authentication passes, domain reputation is healthy, and Klaviyo emails going to spam remains a stubborn Outlook-only problem.

Why shared IPs create this exact problem

Like most large email platforms, Klaviyo sends most customers' mail through shared IP pools. That works well most of the time. Klaviyo manages the pools carefully, and smaller brands get the benefit of an established reputation they couldn't build alone.

The trade-off is that a shared IP means a shared reputation. If another sender on the same IP generates complaints or hits spam traps, Microsoft's filtering can factor that into how it treats everyone sending from that IP, you included. Because Outlook leans on IP reputation, a pool with a patchy history can pull down your Microsoft placement while your domain stays clean everywhere else.

How to tell if this is what's happening

These signals point to an IP issue rather than a domain one:

• The problem is specific to Outlook, Hotmail or Live. Gmail and Yahoo look fine.

• Authentication passes. SPF, DKIM and DMARC alignment all check out.

• Nothing on your side changed. Content, frequency and list hygiene match earlier sends that performed well.

• Google Postmaster Tools shows a healthy spam rate and compliance status. Clean domain signals plus Microsoft-only trouble make the IP the likelier explanation.

How to know if an email really reached the inbox

Klaviyo shows a message as "delivered" whether it landed in the inbox or Junk, so the status alone won't help. Seed tests that include Outlook.com and Hotmail addresses give you a direct read on Microsoft placement. Open rates won't, since Apple Mail Privacy Protection distorts them.

A MailMend on/off A/B test inside Klaviyo. Test results split by provider show whether a problem is Microsoft-only.

What you can actually do about Klaviyo emails going to spam at Outlook

You don't control a shared IP's reputation, but you still have options:

Keep your own sending as clean as possible

Low complaints, engaged segments and consistent authentication won't repair a shared pool, but they reduce your own contribution and help you recover faster.

Ask Klaviyo about dedicated sending if your volume supports it

High-volume senders can sometimes move to dedicated IPs, which removes the shared-reputation risk. A dedicated IP needs enough steady volume to warm up and maintain on its own, so it isn't right for every brand.

Treat Microsoft recipients with extra care

If the problem is isolated to Microsoft domains, be more selective about frequency and targeting for that segment while you work on it. Send your most engaged Outlook subscribers your strongest campaigns first.

Monitor Microsoft separately

Because filtering differs by provider, blending results can hide a Microsoft-only problem for weeks. Track Outlook, Hotmail and Live as their own group in your reporting.

Where this fits in the bigger picture

IP reputation decides a lot at Microsoft. At Gmail, once mail is accepted, there's a different question: Primary or Promotions. MailMend works on that question. It doesn't change your IP reputation. It's built for Klaviyo and designed to improve how often your campaigns and flows land in Gmail's Primary tab, with no changes to your copy or your list, measured in a native Klaviyo A/B test, MailMend on versus off. Book a demo to try it; the free trial starts after your first A/B test.

Frequently asked questions

Why are my Klaviyo emails going to spam only in Outlook?

If authentication passes and Gmail is fine, the most likely cause is the reputation of the shared IP your mail is sent from, which Microsoft tends to weigh heavily.

Can I see my IP reputation with Microsoft?

Microsoft's SNDS tool shows reputation data for IPs you control. On a shared Klaviyo IP you won't have that access, which is why seed testing Outlook addresses matters.

Should every brand move to a dedicated IP?

No. Dedicated IPs need consistent, high volume to build and keep a good reputation. For most brands, clean sending on Klaviyo's shared pools is the better option.