5 Outlook Problems and Issues IT Teams Are Dealing With in 2026

When we first published this post in August 2020, Outlook was Outlook. One Windows app, the same reliable help-desk issues year after year. Clean up the mailbox, disable a bad add-in, rebuild the search index, restore the reading pane. None of that has gone away for the people still on classic Outlook, but it has been almost entirely overshadowed by a bigger problem: there are now two Outlook desktop apps for Windows, Microsoft is migrating everyone to the new one, and the migration keeps shifting underneath IT teams.

Here's the refreshed list for 2026, shaped by what our CSP customers are calling us about.

The State of Outlook in 2026, in One Paragraph

New Outlook for Windows went generally available in August 2024. Small and medium business customers hit the opt-out stage in January 2025, meaning new Outlook became the default with an option to switch back. Enterprise customers were originally scheduled to hit opt-out in April 2026, but Microsoft quietly pushed that to March 2027 in a February 2026 admin center notice, giving enterprises another 12 months of runway. Classic Outlook itself is supported through at least April 2029 for anyone with a Microsoft 365 subscription or perpetual license like Office LTSC 2021 or 2024. That combination (a delayed forced migration plus an extended classic support window) gives IT teams time to plan. It does not give them an excuse to keep kicking the can.

Problem #1: COM Add-Ins Don't Work in New Outlook, and That's a Real Blocker

This is the single biggest reason enterprises are dragging their feet on new Outlook, and it's the reason we recommend most of our customers test this deliberately before the 2027 opt-out lands.

Classic Outlook supports COM add-ins, the deeply integrated extensions that CRM tools, compliance archivers, e-signature platforms, security products, and countless line-of-business integrations have relied on for two decades. New Outlook for Windows does not support COM add-ins at any level. There is no fallback, no compatibility shim, no "reduced functionality" mode. If a critical workflow in your organization depends on a COM add-in today, that workflow breaks the moment a user switches to new Outlook.

The replacement path is web add-ins built on the Microsoft 365 unified add-in model. Many major ISVs already ship these. Many do not, or their web add-ins are feature-limited compared to their COM counterparts. The work isn't hard in any single case. The work is inventorying every add-in across your organization, identifying which users depend on which ones, confirming web-based equivalents exist, and testing them before the switch.

What we recommend:

• Pull a tenant-wide add-in inventory today. You can't plan a migration you haven't measured.
• Categorize each add-in: available as a web add-in with feature parity, available as a web add-in with gaps, no web equivalent yet, no longer in use and safe to retire.
• For any "no equivalent" items, open a support conversation with the vendor now. A year of lead time changes roadmaps. Three months does not.
• Identify users whose workflows still require classic Outlook through 2029, and accept that as a deliberate subset rather than a default.

Problem #2: On-Premises or Third-Party-Hosted Exchange Is a Quiet Dead End

This one almost never comes up in help-desk tickets, which is exactly why it catches people.

New Outlook for Windows treats on-premises Exchange and third-party-hosted Exchange mailboxes like generic IMAP accounts. No calendar integration, no shared mailbox handling, no delegate access the way classic Outlook does it, no Copilot or AI capabilities. If you run hybrid Exchange or you're on a hosted-Exchange reseller, new Outlook is effectively crippled for your users.

The practical implication is that Microsoft's path to the coming AI-enhanced Outlook experience runs through Exchange Online. If you've been putting off migrating the last of your on-prem mailboxes, the new Outlook rollout is a deadline you didn't know you had. (For the on-prem side of the equation, our team has a separate breakdown of what Exchange Server Subscription Edition means for organizations staying on-premises.)

What we recommend:

• Inventory any remaining on-premises or third-party Exchange mailboxes and put them on a migration schedule that completes before March 2027.
• If you're on hosted Exchange outside Microsoft, model the cost of moving to Exchange Online directly. The price-per-user gap is often smaller than people assume once you factor in Copilot-readiness, and your CSP partner can usually provide a like-for-like cost comparison.
• For organizations that can't move (regulatory reasons, sovereignty requirements, specific compliance holds), classic Outlook through 2029 is your bridge, not a permanent answer.

Problem #3: Mailbox Size Limits Still Bite, but the Fix Has Changed

Mailbox limits in the Microsoft 365 world haven't shifted much since 2020. Most Exchange Online mailboxes on business plans still cap at 50 GB. Users on E3, E5, or plans with archiving rights can extend to 100 GB of primary mailbox and add a 1.5 TB auto-expanding archive.

What has changed is the fix. The original 2020 advice was mailbox cleanup followed by archiving to a local PST. That advice is dangerous now. Local PSTs aren't backed up by Microsoft 365, aren't searchable from mobile or web, don't travel with the user, and create compliance blind spots if your organization has data retention obligations.

The modern answer is online archiving: Microsoft 365 archive mailboxes that sit alongside the primary mailbox, are fully searchable, and expand automatically. Most customers on E3 or higher already have the license entitlement and just haven't enabled it. If you're not sure what your current plan includes, our Microsoft 365 licensing assessment is the fastest way to find out what you're already paying for.

What we recommend:

• Check which of your license SKUs include archiving rights (generally E3, E5, Business Premium for smaller orgs, and anything above). Enable online archive for users who need it rather than encouraging PST files.
• Set retention policies that auto-move older mail into the archive rather than relying on users to clean up manually.
• If users are already sitting on piles of local PSTs, plan a PST-to-archive migration as its own project. It takes longer than you'd expect and has real compliance upside.

Problem #4: Search Behaves Differently in New Outlook (and the Old Fix Doesn't Work)

The 2020 version of this post told you to rebuild the Windows search index when Outlook search misbehaved. That advice is specific to classic Outlook, which uses the local Windows Search index for cached-mode mailboxes. It doesn't apply to new Outlook for Windows at all.

New Outlook uses server-side search against Exchange Online. That means search results come from Microsoft's servers rather than a local index, which is faster and more consistent across devices. It also means the failure modes are different. If search is broken in new Outlook, rebuilding the local index will not help. The actual causes tend to be service-side issues with Microsoft Search, account sync problems, or connectivity filters interfering with the search endpoint.

What we recommend:

• For classic Outlook users: the original advice still holds. Rebuild the Windows search index, confirm Outlook is included in the indexed locations, and do it overnight because it takes hours.
• For new Outlook users: check Microsoft 365 service health first, confirm the user is signed in with the right account, and verify network rules aren't blocking the Microsoft Search endpoints. Rebuilding a local index won't do anything.
• For mixed environments, make sure your help-desk runbook distinguishes between the two. We've seen tickets where someone spent four hours rebuilding an index that wasn't being used.

Problem #5: Crashes and Performance Issues Need Version-Specific Triage

In 2020, "Outlook crashes on startup" almost always meant one thing: a bad COM add-in. Disable them all, re-enable one at a time, find the culprit. That's still mostly true for classic Outlook.

For new Outlook, the failure patterns look different because the architecture is different. New Outlook is essentially a desktop wrapper around Outlook on the web, so its failures often look more like browser issues than traditional application issues. Cached-mode behavior, offline support, notification delivery, and keyboard-shortcut handling are all places where new Outlook diverges from classic Outlook in ways that generate tickets.

What we recommend, depending on which client the user is on:

For classic Outlook:

• Start Outlook in safe mode (outlook.exe /safe) to confirm whether an add-in is the issue.
• Run the Office repair tool if the install itself looks damaged.
• Check for a corrupted OST and force a re-sync if cached mode is behaving badly.

For new Outlook:

• Confirm the client is on the latest build. New Outlook updates independently of Microsoft 365 Apps channels.
• Sign out and back in before anything else. Most new Outlook sync issues resolve at that layer.
• Check Microsoft 365 service health. New Outlook shows a wider range of symptoms when backend services degrade than classic Outlook does.
• If a user consistently hits the same issue, have them temporarily switch back to classic Outlook. That still works through at least April 2029, and it's a legitimate contingency while Microsoft continues to close feature gaps.

The Evergreen Fundamentals Haven't Gone Anywhere

Everything in the 2020 version of this post (mailbox cleanup, add-in troubleshooting, deleted-item recovery, search rebuild for classic, view settings) still works where it applied then. None of that advice is wrong. It's just no longer the center of gravity for IT teams running Outlook in 2026.

The center of gravity now is the migration: what breaks when users switch to new Outlook, what AI features they gain, what COM dependencies need replacing, and how to sequence the whole thing before Microsoft's 2027 opt-out removes the default-back-to-classic cushion. That's the conversation we're having with our Microsoft 365 customers every week.

TrustedTech is a Microsoft Solutions Partner for Modern Work, Security, and Infrastructure, delivering licensing, migration, and support for organizations across North America and the UK. For a full view of our Microsoft 365 services, visit our Microsoft 365 Solutions page or explore the Microsoft 365 Resource Hub.