“0x80040115” Connection to Exchange/Sync Lost

Microsoft Outlook users working with Microsoft 365, on-premises Microsoft Exchange, or profile-sync bridges (such as Google Workspace Sync for Microsoft Outlook – GWSMO) may encounter error code 0x80040115.

The error often appears with this message:

Task 'user@domain.com' reported error (0x80040115):
'We can't complete this because we can't contact the server right now. Please try again later.'

The status bar at the bottom of the client typically reports “Disconnected” or “Need Password.”

Quick Answer

Error 0x80040115 indicates that Outlook lost its Remote Procedure Call (RPC) or MAPI-over-HTTP connection to the Exchange server. This is usually caused by an oversized local offline cache file (.ost), network-level packet fragmentation, or a stalled network adapter. Flush your DNS, clear local credentials, and repair or rebuild the .ost file to fix it.

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What the Signal Means

The 0x80040115 hex code corresponds to the MAPI error MAPI_E_NETWORK_ERROR. Outlook returns this code when its communication channel to the mailbox server breaks unexpectedly mid-session, or when it cannot establish an encrypted MAPI-over-HTTP virtual circuit during initial synchronization.

While Outlook reports this as a lost connection, the root cause is often internal: the client’s local cache engine encounters data it cannot synchronize, causing the active synchronization thread to terminate.

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Technical Causes

Error 0x80040115
│
├── Does the error occur across all networks (e.g., home vs. office)?
│   ├── YES ──► Corrupted local .OST file or damaged Outlook profile.
│   └── NO  ──► Network MTU sizing, VPN tunnel dropping, or proxy interference.
│
├── Is the mailbox larger than 20GB?
│   ├── YES ──► Cache exhaustion / indexing stall. Compact or recreate .OST.
│   └── NO  ──► Check for authentication loops or system-level network cache errors.

1. Local .ost File Corruption: If Outlook shuts down unexpectedly while writing updates to disk, internal table indexes within the Offline Storage Table (.ost) can become corrupted. When Outlook tries to sync these broken records, the MAPI layer fails and drops the server session.
2. Network MTU Packet Fragmentation: VPN tunnels and corporate proxy configurations that miscalculate Maximum Transmission Unit (MTU) packet sizes can fragment encrypted TLS packets. MAPI-over-HTTP connections drop fragmented packets, producing immediate sync drops.
3. Stale Windows Identity Tokens: If modern authentication tokens expire without renewing properly, the server rejects session requests, triggering a generic network drop rather than an explicit login prompt.
4. Third-Party Antivirus Add-ins: Security add-ins that inspect network sockets locally can disrupt steady communication streams between Outlook and Microsoft 365.

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Progressive Diagnostic Steps

Step 1: Open Outlook in Safe Mode

Rule out add-in conflicts that might be interfering with your network adapter or MAPI session:

1. Press the Windows Key + R to open the Run dialog.
2. Type outlook.exe /safe and press Enter.
3. Choose your default profile and click OK.

If Outlook connects cleanly and synchronizes without error 0x80040115, an add-in is breaking your connection:

• Go to File > Options > Add-ins.
• At the bottom of the window, select COM Add-ins from the dropdown and click Go.
• Uncheck all third-party add-ins (especially antivirus, antispam, or legacy backup utilities) and restart Outlook normally.

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Step 2: Flush DNS and Reset the Local Network Stack

Stale routing tables and cached DNS records can prevent Outlook from finding the correct Exchange endpoint:

1. Close Outlook.
2. Press the Windows Key, type cmd, right-click Command Prompt, and select Run as Administrator.
3. Run the following commands in order:

ipconfig /flushdns
netsh int ip reset
netsh winsock reset

1. Restart your workstation and relaunch Outlook.

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Step 3: Rebuild the Offline Storage File (.ost)

If network-level fixes do not clear the error, your local cache file is likely damaged. Because Exchange and Microsoft 365 store all data on the server, you can safely delete the local cache and let Outlook re-download your mailbox:

1. Close Outlook completely.
2. Press Windows Key + R, paste the following path into the Run box, and click OK: %localappdata%\Microsoft\Outlook
3. Locate the .ost file named after your email address (for example: john.doe@company.com.ost).
4. Right-click the file and select Rename. Change the extension to .ost.old.
5. Re-open Outlook.

Outlook will create a brand new .ost file and download your mail, folders, and calendar entries directly from the server.

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Step 4: Repair the Outlook Profile

If rebuilding the .ost file does not clear the error, the Outlook profile configuration itself is damaged:

1. Open the Windows Control Panel (switch View to Large icons).
2. Click Mail (Microsoft Outlook).
3. In the Mail Setup window, click Show Profiles.
4. Click Add, name the new profile (e.g., Work Profile New), and configure your email account using modern sign-in.
5. In the bottom section of the window, set Always use this profile to your newly created profile.
6. Click Apply, then OK, and open Outlook.

If you are using Outlook alongside complex hybrid deployments or cross-platform migrations and encounter persistent network rejections, see Fix: “0x80040115” Connection to Exchange Lost for advanced server-side troubleshooting.