Microsoft's 550 5.7.515 Rejection Code Is Showing Up in More Klaviyo Bounce Logs. Here's What's Triggering It

550 5.7.515 is Outlook's rejection for bulk mail that doesn't meet Microsoft's sender requirements. Since May 2025, domains sending more than 5,000 messages a day to Outlook.com, Hotmail and Live addresses must pass SPF, DKIM and DMARC, and mail that fails can be refused with this code. It's a compliance failure on your side, not a bad address, and it will keep happening until the cause is fixed.

Key takeaways

• Microsoft introduced 550 5.7.515 when it began enforcing its bulk sender rules for consumer Outlook addresses in May 2025.

• The code means Outlook refused the message over sender requirements. The address is fine.

• Most email filtering software files this alongside routine bounces, so brands misread a compliance problem as a list problem.

• Microsoft's rules mirror Gmail's, but passing Gmail's checks doesn't guarantee passing Outlook's.

• The rejections don't average out. Every message that fails the checks keeps being refused until the issue is resolved.

If an unfamiliar bounce code has started appearing more often in your Klaviyo reports, there's a good chance it's 550 5.7.515. Microsoft announced its high-volume sender requirements in April 2025, following the path Gmail and Yahoo set in 2024, and began rejecting non-compliant mail soon after. Accounts that never had an Outlook problem started seeing it.

The code doesn't explain itself. It sits in the bounce log next to dead addresses, and unless your email filtering software separates it out, it just looks like the bounce rate ticked up. The real story is that Outlook refused to accept the message.

What the code means

A 550 is a permanent failure: don't retry, this message won't be delivered. The 5.7.515 part says why. Microsoft's own wording is that the sending domain doesn't meet the required authentication level. In other words, the domain failed a requirement Outlook now checks at the moment it decides whether to accept the mail.

A hard bounce from a dead address is a one-off. A 550 5.7.515 is systemic. It hits every message that fails the checks until the root cause is fixed, so a rising count of this one code is a pattern pointing at one cause.

Why more Klaviyo brands are seeing it

Microsoft's rules ask high-volume senders for SPF passing, DKIM passing, DMARC published at least at p=none and aligned with SPF or DKIM, plus working unsubscribe links, valid From and Reply-To addresses, and clean list hygiene. For a while plenty of senders assumed that fixing things for Gmail covered them everywhere.

That assumption is where people get caught. Outlook and Gmail run separate systems, on separate timelines, with separate checks. A domain can satisfy Gmail while still falling short of what Outlook is looking for.

Where the triggers usually are

In the accounts we see, three things come up again and again:

1.  No DMARC record, or one that doesn't align. The From domain and the domain that passes SPF or DKIM don't match, so DMARC fails even though each record exists.

2.  DKIM present but signing with the wrong domain. Common when a branded sending domain was never finished in Klaviyo and mail still signs with a shared domain.

3.  A second tool sending as your domain. A helpdesk, review platform or SMS-to-email service that was never added to SPF or set up with DKIM.

None of these are exotic. They get set up once, work well enough for a long time, and then stop being enough when enforcement tightens.

What good email filtering software should be doing here

The core problem is visibility. If your tools lump this rejection in with ordinary bounces, you won't know it's happening until Outlook delivery has already dropped. You want email filtering software, or at least a bounce report, that separates policy rejections from address problems, shows the raw SMTP response, and flags authentication gaps before Microsoft acts on them.

Most teams only think to check opens and spam complaints. A breakdown that tells you why a message was refused is what catches this early.

A MailMend on/off A/B test inside Klaviyo. Clearing Outlook's checks gets mail accepted; placement is measured after that.

See where your emails are actually landing

A 550 5.7.515 rejection is a compliance problem, solved at the authentication level. Once your domain clears that bar with both Gmail and Outlook, a separate decision is still waiting on each side: does the email land somewhere the subscriber looks?

That's the part MailMend works on. It's built for Klaviyo and designed to improve how often your campaigns and flows land in Primary, working the same way across every send, with no changes to your copy or your list. You measure it in a native Klaviyo A/B test, MailMend on versus off, on clicks and revenue. Book a demo to try it; the free trial starts after your first A/B test.

Frequently asked questions

How do I know if I'm getting this rejection code?

Open the bounce details in Klaviyo and read the raw bounce reason rather than the count. A 550 5.7.515 response is labeled distinctly from a standard hard bounce.

I fixed my Gmail authentication. Why would Outlook still reject me?

Because the two providers enforce separately. The usual culprit is DMARC alignment, or a second tool sending as your domain that Gmail traffic never touched.

Does this affect smaller senders too?

Microsoft's enforcement targets domains sending more than 5,000 messages a day to its consumer addresses. Plenty of Klaviyo brands cross that on campaign days, and the requirements are good practice below it anyway.

Is this the same as landing in the Outlook Junk folder?

No. Junk means Outlook accepted and delivered the message, then filtered it. A 550 5.7.515 means the message was never accepted at all.

Can email filtering software fix the rejection for me?

It can surface it and tell you why. The fix itself is in your DNS and sending setup: SPF, DKIM and an aligned DMARC record for every service sending as your domain.