Outlook Statistics 2026: Users, Market Share, Security & M365 Seats

Outlook has more than 400 million active users worldwide across consumer free webmail and Microsoft 365 commercial deployments, making it the dominant email client inside the enterprise, even as Apple Mail and Gmail control the consumer inbox. Microsoft’s FY25 Q4 disclosure pegged Microsoft 365 Consumer subscribers at 89.0 million, an 8% year-over-year increase, while Microsoft 365 Commercial seat growth moderated to 6% from earlier-quarter peaks.

The data below covers user counts, email-client market share, Microsoft 365 commercial and consumer seat economics, mobile and desktop usage splits, security incidents (Storm-0558, CVE-2025-21298), 2025 outage history, Copilot adoption inside Outlook, and admin-center reporting tools through Q1 FY26.

Key Takeaways

• Microsoft Outlook serves over 400 million active users worldwide across free Outlook.com and bundled Microsoft 365 commercial deployments.
• Microsoft 365 Consumer subscribers reached 89.0 million at fiscal year-end June 30, 2025, up 8% year over year.
• Outlook desktop holds 5.67% of global email opens (Litmus, February 2026), trailing Apple’s 45.51% and Gmail’s 23.54%.
• Microsoft 365 Commercial seat growth slowed from 8% in Q1 FY25 to 6% by Q4 FY25, even as commercial revenue rose by double digits.
• The Storm-0558 token forgery campaign breached approximately 25 organizations, including government agencies, via forged Outlook tokens between May and June 2023.
• CVE-2025-21298 carried a CVSS rating of 9.8, a zero-click Outlook preview-pane RCE that Microsoft patched in January 2025.
• The July 9-10, 2025, Exchange Online incident kept Outlook users offline for roughly 19 hours, the longest single outage of the year.

Editor’s Choice

• Microsoft 365 Commercial revenue grew $10.8 billion or 14% in FY25 Q4, with seat growth and revenue per user driving the lift.
• Microsoft 365 Consumer revenue rose $756 million, or 11%, in the same quarter.
• Outlook Mobile is used on 100+ million iOS and Android devices, according to Microsoft statements summarized in industry coverage.
• Outlook for iOS and Android has more than 30 million monthly users.
• Outlook.com claimed 0.40% of email opens in Litmus’s February 2026 sample, distinct from desktop Outlook’s 5.67%.
• Microsoft 365 Copilot passed 20 million paid enterprise seats in Q1 FY26, embedded across Word, Excel, Outlook, and Teams.
• Apple Mail and Gmail combined account for nearly 90% of the total email-client market share.

Recent Developments

• May 2026: Microsoft 365 Apps shipped Version 2604 (Build 19929.20136) on the Current Channel on May 05, 2026, with a fix for an issue where Outlook closed unexpectedly after replying to a mail item that has labels, one of the routine stability patches Microsoft delivers through the Current Channel.
• November 25, 2025: Microsoft updated Defender for Office 365 to remove leftover calendar entries when security teams perform a Hard Delete, closing a calendar-phishing gap unique to Outlook.
• October 2025: Microsoft began automatically installing Microsoft 365 Copilot for enterprise users, embedding the assistant directly in Outlook alongside Word, Excel, and Teams.
• July 9, 2025: A 19-hour Exchange Online incident beginning at 22:20 UTC affected Outlook on the web, the new Outlook for Windows, and mobile clients.
• March 1, 2025: A “problematic code change” left thousands of Outlook users without email; Microsoft reverted the update at roughly 10:00 PM ET.
• January 14, 2025: Microsoft patched CVE-2025-21298, a zero-click Outlook RCE rated CVSS 9.8, in the January 2025 Patch Tuesday.

Outlook Total Active Users

• Microsoft Outlook anchors a user base of more than 400 million active users across free Outlook.com webmail and bundled Microsoft 365 deployments.
• The product launched as part of Office 97 on January 16, 1997, and has remained Microsoft’s flagship personal information manager for nearly three decades.
• Microsoft’s FY25 disclosure quantified the consumer subscription side: 89.0 million Microsoft 365 Consumer subscribers, up 8%, as of June 30, 2025.
• The 400 million figure spans both paid and free tiers and is reported through Microsoft product pages and aggregator coverage rather than a single 10-K line item.
• Inside the enterprise, Outlook ships as one of the email apps that Microsoft 365 Commercial tenants track via the email apps usage admin report.
• The combined population includes Outlook.com webmail, the Outlook desktop client on Windows and macOS, the Outlook mobile app, and Outlook on the web (OWA) accessed through Exchange Online.

Source: Microsoft Investor Relations, Microsoft product disclosures, Litmus

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Outlook vs Gmail vs Apple Mail

• Apple Mail and Gmail dominate consumer email volume by a wide margin, combined accounting for nearly 90% of total email-client market share in Litmus’s February 2026 reading.
• Apple’s share rests on iPhone defaults plus Mail Privacy Protection’s open-tracking inflation; Gmail’s share rests on consumer signups; Outlook’s share rests on commercial seat licensing.
• Microsoft 365 Consumer’s 89.0 million subscribers represent roughly a fifth of Outlook’s 400 million-plus active user base, with the remainder running on free Outlook.com or commercial seats bundled into business plans.
• Cybersecurity statistics show why enterprise inertia keeps Outlook in place: switching costs around Active Directory, retention policies, and eDiscovery compliance run higher than the per-seat license fee Microsoft charges.

Source: Litmus, Microsoft Investor Relations

Outlook Email Client Market Share

• Outlook holds the third-largest slice of the global email-client market, well behind Apple Mail and Gmail in raw email-open share.
• Litmus’s February 2026 measurement, drawn from over 1.1 billion opens, showed Outlook desktop at 5.67% and Outlook.com at 0.40%.
• The combined Outlook footprint of just over 6% trails Apple’s 45.51% and Gmail’s 23.54%.

By the numbers: Litmus tracked over 1.1 billion email opens in February 2026 and pegged Apple at 45.51%, Gmail at 23.54%, Outlook desktop at 5.67%, and Outlook.com at 0.40%. The Outlook footprint runs primarily on Outlook 2016 and newer client versions in business deployments, where Microsoft 365 bundling consolidates demand.

• The market-share numbers reflect where readers actually open marketing emails rather than seat counts.
• Outlook’s enterprise position remains stronger than the open-share number suggests because corporate inboxes generate fewer marketing-tracked opens per seat than consumer Gmail or Apple Mail accounts.

Microsoft 365 Commercial Seats and Outlook

• Outlook ships in every Microsoft 365 Commercial plan, which makes commercial seat growth a direct proxy for Outlook’s enterprise footprint.
• Microsoft reported a clear deceleration across FY25: 8% seat growth in Q1, 7% in Q2, 7% in Q3, and 6% in Q4.
• Despite the slowdown, Q4 FY25 Microsoft 365 Commercial revenue rose $10.8 billion or 14% year over year.

Key finding: Microsoft 365 Commercial seat growth slowed from 8% in Q1 FY25 to 6% in Q4 FY25, yet Microsoft 365 Commercial cloud revenue rose 15% the same quarter thanks to revenue-per-user gains. The deceleration matters because Outlook is bundled into every commercial seat, a slower seat-add quarter means fewer net new Outlook users entering the enterprise install base.

Outlook in Microsoft 365 Consumer Subscriptions

• The consumer side of Outlook ships through Microsoft 365 Consumer plans (Personal and Family), plus the free Outlook.com webmail.
• Microsoft ended FY25 with 89.0 million Microsoft 365 Consumer subscribers, up 8% year over year.
• Q4 FY25 Microsoft 365 Consumer revenue rose $756 million or 11%, lifted by both subscriber growth and the price increase Microsoft applied in January 2025.
• The free Outlook.com tier sits alongside the paid bundle and contributes to the headline 400 million-user figure.
• Premium Outlook.com features (advanced security, ad-free inbox, expanded mailbox, custom domains) are reserved for paid Microsoft 365 subscribers, creating a clear monetization wedge between the free and paid populations.

Source: Microsoft Investor Relations, FY25 Q4

Outlook Mobile Statistics

• Outlook Mobile carries a meaningful slice of Microsoft’s daily email volume.
• Microsoft has stated Outlook Mobile is used on 100+ million iOS and Android devices, with more than 30 million people using Outlook for iOS and Android every month, a metric Microsoft positions as the active mobile MAU.
• The product itself originated outside Microsoft: Acompli was rebranded as Outlook Mobile on January 29, 2015, following Microsoft’s 2014 acquisition.
• Aggregated industry coverage suggests roughly 60% of Outlook users access email through the Outlook mobile application across iOS and Android, though Microsoft has not published a precise percentage in earnings materials.
• Outlook Mobile competes inside the broader iPhone statistics ecosystem with Apple Mail, the iOS default, and on Android with Gmail’s pre-installed clients.

Outlook Desktop Client Statistics

• The desktop Outlook client commands the largest single piece of Outlook’s measured email-open share.
• Outlook desktop sits at 5.67% of global opens, primarily Outlook 2016 and newer versions, per Litmus’s February 2026 sample.
• Microsoft’s email apps usage admin report tracks per-version adoption inside Microsoft 365 tenants: administrators can view trends over 7, 30, 90, or 180 days and break results down by client (Outlook, Outlook mobile, Outlook on the web, Mac Outlook, Mac Mail) and by Outlook desktop version.
• Microsoft has used the admin report to surface unsupported versions running inside enterprise tenants, allowing IT teams to push upgrades before vulnerabilities like CVE-2025-21298 expose the unpatched fleet.
• The new Outlook for Windows progressive web app, which Microsoft began previewing in May 2022, replaced the legacy Mail and Calendar apps after December 31, 2024.

Source: Litmus, Microsoft Learn (Microsoft 365 admin documentation)

Outlook.com Free Tier Statistics

• Outlook.com is the free, ad-supported webmail client behind a meaningful share of Microsoft’s overall Outlook user count.
• Litmus measured Outlook.com at 0.40% of email opens through web browser access in February 2026, distinct from the desktop Outlook share.
• The free tier underpins the more than 400 million aggregate active user base but trails the paid Microsoft 365 Consumer line of 89.0 million subscribers.
• Microsoft has consolidated its consumer mail surface over the past three years.
• The legacy Windows Mail and Calendar apps were discontinued on December 31, 2024, pushing remaining users toward the new Outlook for Windows progressive web app, which uses the same web stack as Outlook.com.
• The unified surface simplifies feature rollouts and security patches, but also means a single Exchange Online routing-layer failure cascades across more clients at once.

Source: Litmus Email Analytics, Microsoft product documentation

Storm-0558 and Outlook Token Forgery

• The Storm-0558 campaign remains Outlook’s largest disclosed enterprise breach.
• Microsoft’s MSRC advisory confirmed the threat actor targeted the email of approximately 25 organizations, including government agencies and related consumer accounts, between May 15 and June 29, 2023.
• The attacker acquired an inactive MSA consumer signing key and used it to forge authentication tokens for Azure AD enterprise and MSA consumer accounts to access OWA and Outlook.com.

Why it matters: Storm-0558’s blast radius, approximately 25 organizations including government agencies, landed at the heart of the Outlook trust model. The forged tokens worked because a validation issue allowed an MSA-only signing key to be trusted for signing Azure AD tokens, collapsing the boundary between consumer and enterprise authentication for the duration of the campaign.

• Microsoft completed the key rotation and validation fix by late June 2023, but the incident reset enterprise expectations on Outlook key management and accelerated a wider effort to log every Exchange Online API call.
• Detailed cybersecurity attack statistics tracking through recent years show identity-layer breaches as the dominant attack surface across cloud productivity suites.

Source: Microsoft Security Response Center

CVE-2025-21298. Zero-Click Outlook RCE

• The most severe Outlook vulnerability disclosed in the past 18 months arrived in early 2025.
• CVE-2025-21298 carries a CVSS score of 9.8 and is a zero-click remote code execution flaw in Windows OLE, specifically in ole32.dll parsing of RTF-embedded objects.
• Outlook’s preview pane is sufficient to trigger exploitation when a victim views a crafted RTF email, no click required.
• Microsoft addressed the flaw in the January 2025 Patch Tuesday release on January 14, 2025.
• The exploitable surface area was narrow but high-impact: the same preview-pane behavior that makes Outlook fast for triage workflows also opened the zero-click attack vector.
• Mitigation through Outlook’s plain-text mode reduces the risk by disabling automatic RTF rendering, but Microsoft documented the patch as the only complete fix.

Source: Microsoft Security Response Center, NVD

Outlook Outage Frequency

• Outlook users absorbed three notable global outages across calendar 2025.
• On March 1, 2025, a “problematic code change” left thousands of users without email access; Microsoft reverted the update at roughly 10:00 PM ET, and services resumed within an hour.
• A separate Exchange Online disruption in May 2025 followed the same pattern of code-deployment regression, though Microsoft did not publish a single consolidated post-mortem.
• The largest disruption arrived in July: a 19-hour Exchange Online incident beginning at 22:20 UTC on July 9, 2025, and resolved at 5:25 PM UTC on July 10.
• The cumulative downtime profile of 2025 reset enterprise IT expectations on Microsoft 365 service availability.
• The incident affected Outlook on the web, the new Outlook for Windows, and mobile clients connecting to Exchange Online, cutting across every primary client surface at once.
• Companion web browser usage statistics suggest the disruption hit OWA users hardest because browser fallback to Gmail or Apple Mail was harder for corporate accounts.

Source: Microsoft incident statements, Microsoft 365 status page

Outlook Calendar Phishing and Defender Update

• Calendar phishing emerged as a distinct Outlook attack vector during 2025 because the auto-create behavior on incoming meeting invites left calendar entries in place even after security teams deleted the originating email.
• Microsoft addressed the gap on November 25, 2025, when it updated Defender for Office 365 to remove leftover calendar entries during a Hard Delete.
• The update closed a loophole that attackers had exploited by sending malicious meeting invites that lived on past the email’s removal.
• Outlook auto-creates a calendar entry on incoming invites, and users sometimes click invitations directly from the calendar without noticing the source, a behavior pattern that distinguishes Outlook calendar phishing from generic email phishing.
• Defender’s Hard Delete update now connects the email removal action to the linked calendar item, removing both atomically.
• The fix matters most for organizations on Microsoft 365 E5 plans, where Defender for Office 365 P2 is the standard configuration.

Source: Microsoft Defender for Office 365 release notes

Copilot in Outlook Adoption Metrics

• Microsoft 365 Copilot is now embedded in Outlook by default for enterprise users on supported plans.
• Microsoft reported 420 million monthly active Copilot users in Q1 FY26, up from 230 million in Q1 FY25, a roughly 83% year-over-year increase.
• The paid enterprise tier reached 20 million Microsoft 365 Copilot seats in the same quarter.
• Microsoft began automatically installing Microsoft 365 Copilot for enterprise users in October 2025, embedding it directly in Word, Excel, Outlook, and Teams.
• The change shifted the install footprint from opt-in to opt-out, accelerating Copilot’s reach inside Outlook specifically for inbox triage, draft replies, and meeting summarization.
• Enterprise adoption tracking shows 64% of Fortune 500 companies with active Copilot deployments, up from 40% at the end of 2024, the kind of step-change adoption pattern usually seen in new productivity surfaces.
• For broader context on AI-assistant rollouts, see AI agents statistics.

Key finding: Microsoft’s October 2025 auto-install policy moved Copilot from a paid add-on toggle to a default Outlook surface for enterprise tenants. Combined with 20 million paid Microsoft 365 Copilot seats in Q1 FY26, the change positioned Outlook’s compose pane as the primary daily exposure point for AI-assisted writing inside corporate inboxes.

Outlook Email Apps Usage Reporting (Microsoft 365 Admin)

• Microsoft 365 administrators monitor Outlook deployment through a built-in admin-center email apps usage report.
• The report tracks how many email apps connect to Exchange Online and surfaces the version information of Outlook clients in use.
• Default trend windows run 7, 30, 90, or 180 days; selecting a single day pulls data covering up to the last 28 days from the current date.
• Tracked clients include Mac Mail, Mac Outlook, Outlook (desktop), Outlook mobile, and Outlook on the web.
• The “Versions” view inside the report identifies the per-version Outlook desktop user count on Windows, letting IT surface unsupported versions for upgrade.
• Data refresh runs on a 24-48 hour lag, and report exports to Excel CSV are supported for further filtering.

Source: Microsoft Learn (Microsoft 365 admin documentation)

Frequently Asked Questions (FAQs)

Conclusion

Outlook spans more than 400 million active users across consumer and commercial deployments, anchored by 89.0 million Microsoft 365 Consumer subscribers as of fiscal year-end June 30, 2025. The product holds 5.67% of global email opens behind Apple Mail and Gmail, but its enterprise position remains durable thanks to Microsoft 365 Commercial bundling, Exchange Online lock-in, and the October 2025 auto-install of Microsoft 365 Copilot inside Outlook for Word, Excel, Outlook, and Teams users.

The security and uptime record is the variable readers should watch this year and next. Storm-0558 (approximately 25 organizations targeted via forged tokens), CVE-2025-21298 (CVSS 9.8 zero-click RCE in the preview pane), and the 19-hour Exchange Online incident in July together reset enterprise expectations on Outlook reliability and identity-layer trust. Procurement and security teams now factor downtime cost and patch cadence alongside seat economics when evaluating Microsoft 365 plans, and AI integration through Copilot only raises the stakes on the same surfaces.