9 ways to pass Microsoft’s email authentication requirements

Another year, another email security shakeup. And this time, Microsoft is getting in on the action.

As of May 5, 2025, Microsoft officially joined Google and Yahoo in enforcing strict new requirements for bulk email senders. If your messages aren’t properly authenticated with SPF, DKIM, and DMARC, they’re not just headed to spam…they’re being rejected outright.

Unlike past “warning periods,” Microsoft isn’t easing in. This is hard enforcement. If you fail to meet their standards, you’ll likely see bounce codes like 550 5.7.515 in your logs (if you see anything at all).

Email authentication is no longer optional. It’s the foundation of deliverability, reputation, and trust.

Whether you send newsletters, product updates, or transactional emails, following Microsoft’s updated sender requirements is now non-negotiable. Fortunately, it doesn’t have to be complicated, especially if you know where to start.

Below, we’ll walk you through authentication best practices that will help your domain stay compliant, improve inbox placement, and avoid Microsoft’s rejection errors.

Microsoft’s email sender requirements

Microsoft requires bulk senders (anyone sending more than 5,000 messages per day to Outlook.com, Hotmail.com, Live.com, or MSN.com addresses) to follow a set of authentication and sending practices.

Here’s what you need to comply:

• SPF and DKIM must be in place: You need to authenticate your email using both Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM).
• DMARC record with p=none (or stronger): Your domain must have a published DMARC policy. Sure, p=none meets the minimum, but we recommend moving to quarantine or reject.
• SPF or DKIM must align with the From domain: At least one method (SPF or DKIM) must align with the domain used in the “From” address. Ideally, both should.
• Valid From and/or reply-to address: You must use a From or Reply-To address that can receive messages. Dead-end inboxes don’t work.
• Unsubscribe functionality: Marketing or bulk messages must include an obvious, working unsubscribe option to let users opt out easily.
• Transparent sending practices: Don’t use deceptive headers. Only send to users who’ve consented.

9 authentication best practices to pass the requirements

1. Publish a DMARC record and enforce alignment

DMARC (Domain-based Message Authentication, Reporting & Conformance) tells mailbox providers how to handle emails that fail SPF or DKIM checks. It’s a core requirement for Microsoft deliverability.

However, publishing a DMARC record isn’t enough on its own. You also need alignment, which means your SPF or DKIM must match the domain in your visible “From” address. Without alignment, DMARC will fail (even if SPF or DKIM technically passes).

Start with p=none for visibility, then work toward quarantine or reject for full protection.

2. Include all sending sources in SPF

Your SPF record tells mailbox providers which IP addresses and services are authorized to send email on behalf of your domain. If even one of your legitimate senders is missing, Microsoft could reject your mail.

This happens more often than you’d think. Marketers spin up a new tool, like a CRM or newsletter platform, but forget to update SPF. Suddenly, Microsoft sees that email as unauthorized.

Audit your SPF record regularly and include every platform that sends on your behalf. 

Remember: SPF has a 10-lookup limit, so keep things tidy with mechanisms like include: to stay compliant. You can also use a service like Valimail Enforce to overcome these limitations.

3. Use DKIM on every outbound email

DKIM adds a digital signature to your messages to verify that the content hasn’t been tampered with in transit. It proves that a message really came from you.

For Microsoft (and other major providers), DKIM is required for bulk senders. If your emails aren’t signed with DKIM (or if the signature fails), your messages will likely be rejected outright.

Make sure every sending platform you use is set up to sign with DKIM and that the domain in the signature aligns with your visible From address.

4. Segment third-party tools to subdomains

Using multiple tools to send email? Great. Just don’t let them all send from your root domain. Instead, assign each tool a dedicated subdomain like news.yourdomain.com or support.yourdomain.com.

This gives you more control over SPF, DKIM, and DMARC for each sender. It also keeps your primary domain’s reputation clean. If something breaks or gets flagged, it won’t take your entire domain down with it.

This simplifies alignment, too. You can align each subdomain independently and spot issues faster in DMARC reports.

5. Regularly audit DNS and mail flows

Email infrastructure isn’t “set it and forget it.” Teams inevitably add tools, migrate platforms, or change vendors, and your DNS records and mail flows can drift out of alignment.

That’s why you need regular audits:

• Check your SPF records for old or missing includes. 
• Make sure DKIM is active and aligned. 
• Review your DMARC policy and make sure it still reflects your goals.

Also audit your mail flow: who’s actually sending email using your domain? If you don’t recognize a source, dig deeper sooner rather than later.

6. Monitor performance with DMARC reports

Yes, DMARC is primarily about enforcement, but it’s also about visibility. DMARC reporting gives you a behind-the-scenes look at how your emails are being authenticated and which sources are passing (or failing).

Valimail Monitor makes this easy by translating raw DMARC data into a clean dashboard. You’ll quickly see which senders are failing alignment, missing DKIM, or sending without authorization.

This is the fastest way to catch configuration issues before they result in a 550 5.7.515 error (and to prove you’re on track with Microsoft’s requirements).

7. Adopt a DMARC enforcement policy

Once you’ve confirmed that your legitimate senders are passing authentication and alignment, it’s time to upgrade your DMARC policy.

Start with p=none to gather data, then move to quarantine or reject. These enforcement settings tell mailbox providers to block or filter unauthorized messages. This protects your domain from spoofing and helps you comply with Microsoft’s requirements.

Enforcement is what turns DMARC from a monitoring solution into an active defense system. And Microsoft (like Google and Yahoo) expects serious senders to take that step.

Curious what your DMARC policy is currently set at? Use our free domain checker to get more insights into your domain:

8. Warm up new IPs and domains slowly

Planning to launch a new campaign or switch to a new sending platform? Don’t go from zero to 50,000 emails in a day.

Mailbox providers (including Microsoft) watch for sudden spikes in volume, especially from new IPs or domains. That’s typically what a spammer or phisher would do, right? Big jumps look suspicious and can trigger throttling, delays, or outright blocks.

Instead, gradually increase your sending volume over time. This “warming up” period helps establish a positive reputation and builds trust with Microsoft’s filters.

9. Avoid spammy content and keep complaint rates low

Even perfect authentication won’t save you if your emails look like spam. Microsoft’s filters weigh content quality and user engagement alongside authentication.

Avoid deceptive subject lines, misleading preview text, or overuse of sales-y language. Make sure your branding is clear and your unsubscribe link works.

Also, keep an eye on complaint rates. If too many recipients report your emails as spam, it can tank your sender reputation, and that makes deliverability harder (no matter how well your DNS is set up).

Authenticate with confidence

Microsoft’s new requirements aren’t just a policy update. They’re a wake-up call for senders to get serious about authentication. Following best practices like aligning SPF, DKIM, and DMARC is how you protect your brand and guarantee emails actually arrive.

If you’re unsure where you stand, start with visibility. Don’t wait for a 550 5.7.515 error to find out something’s broken. Valimail Monitor is a free solution that shows you exactly which services are sending on your behalf and whether they’re passing authentication.