Outlook IMAP Account Not Syncing: Causes and Fixes

Outlook can connect to almost any IMAP account—Gmail, Yahoo, FastMail, a business mailbox hosted on cPanel—and most of the time that connection holds without attention. When Outlook IMAP stops syncing, the failure usually looks the same: the inbox sits frozen at whatever state it was in when sync last ran, new mail does not appear, folders that should be there are missing, or Outlook keeps prompting for a password it already has. The culprit is almost never the network itself.

IMAP in Outlook breaks in specific ways, and each way has a specific fix. The most common complaint—folders that exist on the server but are invisible in Outlook—takes under two minutes to resolve once you know where to look. The trickier failures, like a stale local data file or a changed authentication requirement, take longer but follow a clear path.

One scope note before we start: this guide covers Outlook IMAP not syncing specifically—not Exchange, not Microsoft 365-connected mailboxes, and not the new Outlook's native Gmail sync. Those protocols fail differently. If your account folder pane shows a cloud icon or account settings say 'Connected to Microsoft Exchange,' this guide does not apply to that account.

Why is my Outlook IMAP account not syncing?#

Four causes cover the large majority of Outlook IMAP sync failures. Identifying which one applies tells you exactly which fix to run—rather than trying everything in sequence and hoping one sticks.

Before you start: two checks worth doing first#

Two quick checks before touching any Outlook settings can save you from running the right fix on the wrong problem.

First, confirm that IMAP is enabled on the provider side. Most major providers—Gmail, Yahoo, Outlook.com, FastMail—default to IMAP on, but some disable it after a period of inactivity or after a security event. Log in to your email provider's webmail interface, open its settings, and verify IMAP access is toggled on. For Gmail this is under Settings > See all settings > Forwarding and POP/IMAP.

Second, confirm the connection uses port 993 with SSL/TLS, not port 143. The older unencrypted port sometimes causes silent sync failures on providers that have tightened security. You can check this under File > Account Settings > Account Settings > select the account > Change > More Settings > Advanced in Outlook for Windows, or under Tools > Accounts > Advanced in Outlook for Mac.

How to fix Outlook IMAP not syncing#

Run these steps in the order shown. The first fix—subscribing to folders—solves the most common problem. The remaining steps address the inbox itself not syncing, not just individual folders being absent.

1. 1

   Step 1: Subscribe to missing IMAP folders

   In Outlook for Windows (classic): right-click the account in the folder pane and select IMAP Folders. Click Query to fetch the server's current folder list. Find the folders that are absent, select them, and click Subscribe, then OK. In Outlook for Mac: go to Tools > IMAP Folders, select the folders, and click Subscribe. Subscribed folders appear in the pane within seconds. This is the standard fix for 'why are my IMAP folders missing in Outlook'—the server has the folders; Outlook simply was not told to display them.

2. 2

   Step 2: Delete and rebuild the OST file

   The OST file is Outlook's local cache of your mailbox. When it becomes stale or corrupted, sync can stall even though the server is healthy. Close Outlook completely before touching this file. On Windows, navigate to C:\Users\[your name]\AppData\Local\Microsoft\Outlook\ and find the .ost file for the affected account. Delete it or rename it with an .old extension. Reopen Outlook—it detects the missing file, creates a fresh one, and syncs from the server. Progress appears in the Send/Receive status bar at the bottom of the Outlook window. No mail is lost; the server is always the source of truth.

3. 3

   Step 3: Generate and save an app password

   Several providers—Gmail with two-step verification on, Yahoo, and Outlook.com with two-factor authentication enabled—no longer accept a regular account password over IMAP. They require an app password: a one-time credential generated in the provider's security settings specifically for third-party mail clients. In Gmail: Google Account > Security > How you sign in > 2-Step Verification > App passwords. In Yahoo: Account Security > Generate app password. Copy the generated string, return to Outlook's account settings, and replace the saved password with the app password. This resolves the credential loop that the 'app password required IMAP Outlook' prompt describes.

4. 4

   Step 4: Reduce IMAP concurrent connections

   Some IMAP servers—particularly smaller hosted providers—enforce a limit on how many simultaneous connections a single client can open. When Outlook exceeds that limit, the server drops some connections and partial sync results: certain folders update while others freeze. In Outlook for Windows: File > Account Settings > Account Settings > select the account > Change > More Settings > Advanced. Set 'Maximum number of server connections to use' to 1 or 2, then click OK. Outlook reconnects using fewer sessions and the blocked folders begin syncing. If Sent Items are duplicating, also check the Advanced tab and tick 'Do not save copies of sent items,' then map the Sent Items path to the server folder labeled 'Sent' or 'Sent Items.'

How these fixes differ across Outlook versions#

The four fixes above apply across Outlook versions, but the menus that lead to each setting differ depending on which version and platform you are running. Use this table to find the right path.

What to do when Outlook IMAP still will not sync#

If the four steps above did not resolve the sync failure, one of three things is usually happening: the account itself is misconfigured, the provider has changed its authentication requirements, or a problem in the Outlook profile is blocking the connection for all accounts.

Remove and re-add the account. This is the most reliable reset for a stubborn IMAP sync. In Outlook for Windows: File > Account Settings > Account Settings > select the account > Remove. Then add it back: File > Add Account, enter the email address, and let the autoconfigure run. If autoconfigure does not find the correct settings, enter the IMAP server details manually—incoming port 993 with SSL/TLS, outgoing port 587 (STARTTLS) or 465 (SSL)—from your provider's documentation.

Create a new Outlook profile. A corrupted Outlook profile can prevent any IMAP account from syncing, even accounts that were working before. Go to Control Panel > Mail > Show Profiles > Add, create a new profile, and add the IMAP account to it. Open Outlook using the new profile and test sync from a clean state.

Check for a provider-side security alert. Some providers suspend IMAP access after detecting unusual access patterns or after a security event. Log in to the webmail interface and look for a security or activity notification. Google sometimes shows a 'critical security alert' that blocks third-party access until confirmed from the account's security page at myaccount.google.com/security.

A faster way to manage IMAP accounts across every inbox#

The steps above fix the specific failure that sent you to this page. But IMAP sync in Outlook is recurring maintenance: OST files grow stale, providers update their authentication requirements, folder subscriptions reset after an account removal. If you manage more than one IMAP account—or mix IMAP with Gmail and Outlook.com—this cycle repeats across multiple clients on an ongoing basis.

AI Emaily connects IMAP accounts alongside Gmail, Outlook, and other providers in a single client, using server-side sync that does not rely on a local OST file. When a provider switches to requiring an app password, AI Emaily prompts a one-time reauthorization rather than looping on a broken credential. Folder subscription is automatic—every folder on the server appears without a manual subscribe step, and there is no local cache to rebuild or repair.

We build AI Emaily. It is available at aiemaily.com with a 7-day free trial; the IMAP connection setup guide is at aiemaily.com/docs/connect-imap, and plan details are at aiemaily.com/pricing.