New Outlook Migration: What Breaks and How to Prepare

Microsoft is replacing classic Outlook with a web-based version called “new Outlook,” and the transition is not optional. Every Microsoft 365 business subscriber will eventually lose access to the desktop Outlook they’ve used for decades. The enterprise opt-out deadline was pushed from April 2026 to March 2027, but classic Outlook support ends entirely in April 2029. If your business depends on Outlook plugins for CRM, e-signatures, compliance archiving, or email encryption, you have a narrow window to audit what will break and find replacements.

This is not just a visual refresh. New Outlook is a fundamentally different platform built on web technologies, and it drops support for COM add-ins entirely. That single change will disable tools that many businesses use dozens of times per day without thinking about them.

What Is Actually Changing

Classic Outlook is a thick desktop application that runs locally, stores email in cached Exchange mode, and supports COM add-ins. These add-ins are compiled Windows programs that integrate directly into Outlook’s interface and have deep access to email data, calendars, and contacts.

New Outlook is a web application wrapped in a desktop shell. It connects to Microsoft’s cloud servers and renders everything through a browser engine. The underlying architecture is the same engine that powers Outlook on the web, which means it only supports web-based add-ins built on JavaScript and the Office Add-in platform. COM add-ins, which have been the standard integration method for enterprise email tools for over 20 years, do not load at all.

Microsoft has been migrating consumer Outlook users since 2024. Enterprise customers have been able to opt out, but that option expires in March 2027. After April 2029, classic Outlook stops receiving security updates entirely, making it a compliance liability for any business subject to HIPAA, PCI DSS, or CMMC requirements.

Which Tools Will Break

The COM add-in removal is the biggest operational risk. Here is what stops working when your organization switches to new Outlook.

CRM integrations. Salesforce for Outlook, the HubSpot Outlook sidebar, and Dynamics 365 App for Outlook all rely on COM add-ins in their classic forms. Salesforce has been pushing users toward its Lightning Sync and browser-based integration for several years, and HubSpot offers a web add-in alternative. However, feature parity varies. Sales teams that use one-click email logging, contact record sidebars, or meeting scheduling through these plugins need to test the web versions thoroughly before switching. Some features may be missing or work differently.

E-signature tools. DocuSign for Outlook and Adobe Sign both offer COM-based plugins that let users send documents for signature directly from an email. Both vendors have web add-in versions, but the migration is not automatic. Your existing DocuSign or Adobe Sign plugin will simply disappear from the Outlook toolbar after the switch. Users will need to install the web add-in replacement and may need reconfiguration.

Compliance and archiving tools. Email archiving solutions from vendors like Barracuda, Mimecast, and Global Relay often use COM add-ins to provide one-click archiving buttons, legal hold notifications, or compliance disclaimer insertion. If your industry requires email retention (financial services, healthcare, legal), verify that your archiving vendor has a web add-in available. If they don’t, you need to find an alternative before migration.

Email encryption. Plugins from Virtru, Zix (now OpenText), and similar providers that add encryption buttons to the Outlook compose window are COM-based. Some have migrated to web add-ins, others have not. Without the plugin, your employees lose the easy “encrypt this email” button and may stop encrypting sensitive communications out of friction.

Industry-specific plugins. Practice management software in legal and healthcare, accounting integrations, and project management tools often include Outlook plugins built on COM. These are the easiest to overlook during an audit because they’re specific to your industry and may come from smaller vendors who are slower to release web add-in replacements.

Beyond Add-Ins: Other Changes That Affect Daily Work

The add-in breakage gets the most attention, but several other changes in new Outlook affect how your team works.

Offline access is reduced. Classic Outlook uses cached Exchange mode, which downloads your entire mailbox locally. You can read, compose, and organize email without an internet connection, and everything syncs when you reconnect. New Outlook relies primarily on a server connection and offers limited offline functionality. For employees who travel frequently, work from client sites with unreliable Wi-Fi, or operate in areas with spotty connectivity, this is a meaningful downgrade.

PST file support is limited. Many businesses use PST files to archive old email locally or to import historical mailboxes. New Outlook does not support PST import or export in the same way classic Outlook does. If your retention policy relies on local PST archives, you need an alternative approach before migrating. Microsoft’s recommended path is cloud-based archiving through Exchange Online Archiving, which is included in some Microsoft 365 plans but not all.

On-premises Exchange gets second-class treatment. If your organization still runs Exchange Server on-premises rather than Exchange Online, new Outlook treats your mail server with reduced functionality. Calendar features, room booking, and advanced search capabilities that work fully with Exchange Online may be limited or unavailable with on-premises Exchange. Microsoft is clearly steering businesses toward Exchange Online, and the new Outlook experience reflects that priority.

How to Audit Your Outlook Dependencies

Your IT provider should be doing this audit for you. If they haven’t brought it up yet, that’s a conversation worth having this quarter. Here is what a thorough Outlook dependency audit covers.

Step 1: Inventory every COM add-in across your organization. Don’t rely on asking employees what they use. Pull the add-in list from each machine programmatically using your endpoint management tool (Intune, ConnectWise, or your MSP’s RMM platform). Every installed COM add-in for Outlook should appear in the report. Group them by vendor and function.

Step 2: Check each vendor for a web add-in replacement. For every COM add-in in your inventory, visit the vendor’s documentation and confirm whether a web add-in (Office Add-in) equivalent exists. Note the feature differences. Some web add-ins cover 90% of the COM version’s functionality; others cover 50%. Document what’s missing so your teams know what changes to expect.

Step 3: Identify add-ins with no replacement. This is the critical list. If a vendor has no web add-in and no public timeline for releasing one, you have three options: find an alternative product, build a workaround, or delay migration for those users. Your IT provider can help evaluate alternatives, but the decision about which tools to replace involves the business users who depend on them.

Step 4: Test new Outlook with a pilot group. Deploy new Outlook to 10 to 15 users across different departments. Include at least one user from every team that relies heavily on Outlook add-ins: sales (CRM), legal or finance (compliance archiving), and anyone who sends documents for signature regularly. Run the pilot for at least two weeks and track every issue.

Step 5: Document your migration timeline. Based on the pilot results, set realistic dates for each department’s migration. Teams with fully supported web add-ins can move first. Teams waiting on vendor replacements go last. Build in buffer time because vendors regularly miss their own release dates.

Your Timeline and Decision Points

The March 2027 enterprise opt-out deadline feels distant, but the preparation work has real lead time.

Now through Q3 2026: Run the add-in audit. Identify what breaks and what has replacements. Start vendor conversations for anything without a web add-in alternative. This is also the right time to evaluate whether your on-premises Exchange environment should move to Exchange Online before the Outlook migration compounds the complexity.

Q4 2026 through Q1 2027: Deploy web add-in replacements as they become available. Run your pilot group on new Outlook. Train affected users on workflow changes, especially sales teams losing familiar CRM sidebar features.

Q1 2027: Finalize migration for all users before the opt-out deadline. Ensure your email security stack works correctly with new Outlook, since some email filtering and phishing protection tools may also need updates.

If your business has already moved to Microsoft 365 and Exchange Online, the transition will be smoother because new Outlook is built for that environment. If you’re still running on-premises Exchange, the Outlook migration becomes part of a larger cloud migration conversation that should start now.

The most expensive version of this migration is the one where you find out what broke after Microsoft flips the switch. CRM data stops syncing, signatures pile up unsigned, and compliance archives go dark while your team scrambles to find web add-in replacements they’ve never tested. The audit takes a few hours. The unplanned outage takes days. If your managed IT provider hasn’t raised this topic with you yet, bring it to your next meeting. This is exactly the kind of platform change that a proactive IT partner should be tracking and planning for on your behalf.