A Practical Guide to Improve Email Deliverability in Outlook

With over 400 million active users, Outlook stands strong as the second-largest email platform after Gmail. From a user’s perspective, Gmail often wins attention for its AI-driven spam filtering and reliable deliverability. 

Yet, millions still prefer Outlook for its comprehensive email ecosystem for its smart filtering, seamless integrations, and an intuitive interface.

But there’s a catch: Outlook also enforces some of the strictest authentication and filtering standards in the industry. Unlike Gmail, Microsoft evaluates identity consistency far more aggressively.

DKIM d= alignment, Return-Path alignment, and overall domain coherence carry significant weight in Outlook’s filtering logic. Tenant-level behavior across Microsoft 365 environments also influences trust, which is why even small alignment issues can trigger suppression.

That’s where most senders struggle.

Nailing email deliverability on Outlook can be notoriously tricky because even minor compliance gaps can trigger throttling, filtering, or outright blocking. This blog presents a structured, step-by-step framework designed to improve your email deliverability on Outlook.

7 Steps to Improve Outlook Email Deliverability 

Outlook doesn’t gradually warn you before filtering. It suppresses first and explains later, which is why deliverability fixes must be structural. Here’s how to get started:

1. Authenticate Your Domain with SPF, DKIM, and DMARC

Similar to Gmail, Outlook first evaluates a sender at the identity layer before assessing reputation, engagement, or email content. Microsoft enforces identity checks more strictly, especially around DKIM d= alignment and Return-Path coherence.

Verifying email authenticity depends on three essential mechanisms:

• SPF (Sender Policy Framework): It specifies which mail servers are allowed to send email on behalf of your domain
• DKIM (DomainKeys Identified Mail): It cryptographically signs each message so Outlook can verify if the email was altered in transit and it belongs to a specific domain
• DMARC (Domain-Based Message Authentication, Reporting, and Conformance): It ensures the domain in the From address aligns the domain that passed SPF or DKIM

Gmail may tolerate minor DKIM domain mismatches. Outlook doesn’t.

A DKIM signature that passes but uses a different d= domain (for example, your ESP’s domain rather than your own) is treated as an ambiguous identity, which can result in silent tenant-level suppression, even when SPF, DKIM, and DMARC all show “pass.”

If your outreach platform signs DKIM using its own domain (for example, d=sendgrid.net or d=mailgun.org), Outlook often downgrades trust. Therefore, always configure DKIM to sign with your primary domain or a first-party subdomain you control.

This step centers on one question:

“Does Outlook fully authenticate your message on receipt?”

If SPF, DKIM, or DMARC fail in message headers, every downstream deliverability fix becomes unstable. Therefore, to verify this in practice:

• Publish valid SPF, DKIM, and DMARC records for the exact domain used in your From address
• Send a test message to an Outlook or Microsoft 365 inbox
• Open the full message headers and confirm:
  
  • SPF = pass
  • DKIM = pass
  • DMARC = pass

In addition, please remember that passing SPF, DKIM, and DMARC is the baseline. Outlook requires an additional layer: identity alignment.

Even when all three authentication checks pass, Outlook may still reduce trust if the domains in your sender identity do not match. This is where Microsoft evaluates domain coherence using the From address, the DKIM d= domain, and the Return-Path.

To learn more, read our guide on the best cold email deliverability strategy.

2. Analyze Outlook Reputation with SNDS and JMRP

After authentication and domain alignment, reputation checks help confirm whether Outlook trusts your sender identity. Smart Network Data Services (SNDS) and Junk Email Reporting Program (JMRP) can provide limited visibility.

But they apply only to Outlook.com/Hotmail and don’t reflect how Microsoft 365 business tenants evaluate your domain.

Therefore, if you send meaningful volume to consumer Outlook addresses, you can add your IPs to SNDS and enroll them in JMRP to receive complaint data.

For most B2B senders, these tools are optional because they don’t offer insight into Microsoft 365 inbox placement. That’s where a deliverability platform becomes essential.

MailReach’s email warm-up tool provides per-mailbox reputation insights, which is deeper than traditional campaign-level metrics.

Each connected inbox is assigned a live reputation score that can be reviewed at the mailbox, domain, and provider level, including segmentation for Outlook and Google Workspace. This keeps reputation analysis separate from engagement metrics (for example, opens and replies).

The platform also provides real-time notifications via Slack and webhooks. These alerts are triggered by reputation score drops or recoveries, enabling teams to respond before delayed systems such as SNDS surface enforcement actions.

3. Fix Bounce and Complaint Issues Before Scaling Volume

In Outlook environments, reputation damage is primarily caused by hard bounces, spam complaints, and lack of engagement. Volume only becomes relevant after these issues are resolved. Until these signals are corrected, any increase in sending volume will amplify negative reputation indicators.

a. Start by fixing hard bounces

Permanently suppress any email address that returns a hard-bounce message, such as::

• “User unknown”
• “No such recipient”
• “Mailbox unavailable”

Don’t retry these addresses or attempt to send to them again using a different domain. As a general rule: keep your total bounce rate consistent below 3%.

Outlook often demotes senders to the “Other” tab before moving them to the Spam folder. If your open rate for Outlook recipients drops below ~2%, it signals early deterioration in tenant-level trust.

b. Minimize complaint exposure

Remove long-term inactive subscribers from your mailing list. These are recipients who:

• Never reply to any of the campaigns
• Never open any messages across full follow-up sequences
• Reappear after being re-queued from legacy lists

In enterprise Outlook tenants, even a small number of complaints carries disproportionate weight. Once a tenant links your sender identity to complaints, subsequent sends will face reduced trust.

To prevent this, perform the following checks before any address enters Outlook-driven outbound campaigns

• Validate email syntax (correct format and domain)
• Confirm mailbox existence (via SMTP probe or verification API)
• Check MX behavior (ensure MX records resolve and accept mail)

c. Rebuild your email list with nominative, role-aligned recipients

Source only one verified decision-maker per role (with a corporate-domain email address), and tailor the message to that role’s responsibility.

Test every new data source with a micro-batch (fewer than 50 contacts) and only scale sources that generate genuine replies and maintain minimal complaint rates.

To learn more, read our guide on how to prevent emails going to spam.

4. Warm Up Outlook Sending Patterns and Manage Volume

Until now, the focus has been on validation and cleanup. But this step initiates execution, gradually introducing controlled volumes to show Outlook that your sending patterns are reliable and safe.

Warm-up isn’t about inflating opens or sending arbitrary low volumes; it’s about generating real engagement from inboxes Outlook already trusts, especially Microsoft 365 and Google Workspace.

a. Define your warm-up path

Base the warm-up path on your current reputation. For example:

• A new domain or fresh sending identity typically requires a minimum of 14-day controlled ramp-up.
• A domain with prior Outlook suppression usually requires 3 to 4 weeks of consistent behavior before trust begins to normalize.
• Heavily damaged domains may require longer recovery, depending on prior complaint activity.

Once warm-up begins, it should continue indefinitely. Outlook reputation decays during idle periods, so stopping warm-up can slow recovery or trigger new filtering patterns.

Accelerating either one increases the risk of early throttling, ‘Other’ tab placement, and tenant-level filtering.

Before scaling, set strict daily volume caps that apply across all sending systems (sales tools, marketing platforms, support systems, and transactional SMTP).

b. Distribute sends evenly across time

Outlook enforces rate limits that are highly sensitive to high-frequency sending behavior.

Here’s what you can do:

Avoid

• Launching campaigns at the top of the hour
• Multiple platforms sending simultaneously
• Releasing large batches within short time windows

Instead

• Spread sends evenly across business hours
• Maintain a consistent daily cadence
• Keep per-hour volume flat

This reduces 421 deferrals, graylisting, and tenant-level delivery delays.

After introducing volume, Outlook applies behavioral tests to assess your sending thresholds.

Pause further expansion immediately if you observe any of the following:

• New throttling events
• Repeated delivery delays
• Sudden drops in Outlook engagement metrics (opens, clicks, or replies)
• Consistent placement in the “Other” across multiple tenants

Behavior is considered stable only after meeting all of the following:

• At least 7 consecutive days without throttling
• Consistent delivery timing across tenant types
• No fresh complaint activity from Outlook users

Only after this baseline holds should you expand daily volume further.

5. Optimize Email Content for Outlook Spam Filters 

Once traffic is established, focus on controlling four content-layer risks that can trigger Outlook filtering, even if sender identity and sending behavior are fully compliant.

a. Simplify links and tracking

Links are one of the first causes of content-related spam filtering on Outlook, especially when redirects or tracking parameters are involved.

In fact, Corporate Outlook tenants aggressively inspect links through Advanced Threat Protection (ATP). Long UTM strings, multiple redirects, or mismatched link text can trigger security scanning and reduce inbox placement.

Tracking increases filtering probability because it resembles automated campaigns. Many B2B cold outreach senders remove link tracking entirely and rely on reply rate as their primary performance metric.

Avoid high-risk redirect chains like: trk.yourtool.com → bit.ly → yoursite.com

Prefer:

• Direct destination URLs: https://yourcompany.com/demo
• Short, internal parameters: /demo or ?ref=outreach
• One redirect at most, never chained redirects

Clean link structure lowers the likelihood of filtering, ATP warnings, or partial suppression.

b. Remove unnecessary UTM parameters from outreach links

Bulk-style UTM parameters make emails look like automated campaigns rather than personalized communication.

For instance, a high-risk link looks like: 

?utm_source=outreach&utm_medium=seq3&utm_campaign=q3_pipeline&utm_content=linkA. 

On the other hand, safer alternatives look like “/demo or ?ref=outreach.” Keep links minimal and human-like. Avoid anything that resembles marketing automation.

c. Avoid attachments in initial cold emails

Attachments in first-touch B2B cold outreach are highly risky. Doing so may trigger Outlook’s threat detection and malware scanning. Share files only after the recipient has engaged and trust has been established in the thread.

d. Eliminate phrases linked to spam behavior

Although Outlook does not use traditional “spam keyword lists,” certain categories of wording still increase filtering risk because they correlate with historical spam patterns.

High-risk categories include financial terms, health-related claims, aggressive growth promises, and anything resembling promotional or consumer marketing language.

6. Test Your Deliverability With Outlook

After completing the earlier steps, run a spam test to confirm your Outlook inbox placement. You can use the free MailReach Spam Test to check authentication, blacklist exposure, link safety, HTML structure, and content-related risks.

The test evaluates SPF, DKIM, and DMARC authentication, blacklist exposure, HTML structure, link safety, and content-risk indicators. This occurs before any escalation or delisting request.

The tool can also perform recurring automated spam testing. Slack and webhook alerts notify teams of any placement changes, allowing them to monitor filtering behaviors across multiple Microsoft 365 tenants.

How MailReach Supports Outlook Delivery Control

Outlook inbox placement doesn’t fail abruptly. It is usually throttled, demoted, and suppressed in stages across different Microsoft 365 tenants before full blocking occurs. By the time most teams react, Microsoft has already classified the sender and adjusted the domain reputation.

While you should diligently apply the seven steps outlined in this blog, these efforts become far more reliable with continuous verification.

A dedicated deliverability platform like MailReach runs warm-up across real Microsoft 365 and Google Workspace inboxes. It tracks provider-level sender reputation per mailbox, and validates placement directly inside Outlook across Inbox, Other, and Spam folders.

This allows Outlook behavior to be measured during setup, recovery, and scaling, not just after problems arise

‍Run a free Spam Test on MailReach to verify your Outlook inbox placement.