How to Fix Email Sync Issues in Outlook and Gmail

Email sync failures are a top-five IT support ticket. Emails arrive on your phone but not desktop. Calendar invites vanish. Sent messages stay in the outbox. This guide covers root causes and exact fixes for Outlook, Gmail, and Exchange.

Understanding Sync Protocols

• Exchange/Microsoft 365 - Full bidirectional sync (email, calendar, contacts, tasks). Most reliable when working, most complex when broken.
• IMAP - Email sync only. Used by Gmail, Yahoo, most providers.
• POP3 - Downloads to one device. Not true sync. Switch to IMAP to fix most issues.

Quick Diagnosis

• No new emails - Connection or authentication problem
• 30+ minute delays - Sync frequency or push notification issue
• Phone but not desktop - Configuration mismatch or cached mode
• Calendar not syncing - Exchange/O365 specific
• Stuck in outbox - SMTP authentication failure

Fix 1: Force Manual Sync

Outlook: Press F9 or Send/Receive All Folders. Watch the status bar for "Connected" or "Disconnected."

Gmail: Ctrl+Shift+R on web, pull down on mobile to force refresh.

Fix 2: Re-authenticate

Modern email uses OAuth2. When tokens expire, sync stops silently.

1. File, Account Settings, select account, click Repair
2. For M365: sign out and back in to refresh tokens
3. For Gmail in Outlook: re-authorize at Google security settings

Fix 3: Check Mailbox Storage

• M365: 50 GB default. File, Info, Mailbox Cleanup
• Gmail: 15 GB shared across Gmail, Drive, Photos

Fix: archive old emails, empty Deleted Items/Junk, or request quota increase.

Fix 4: Rebuild Outlook Profile

1. Close Outlook
2. Control Panel, Mail, Show Profiles
3. Add a new profile, enter email, let auto-discover configure
4. Set as default, open Outlook, allow full resync

Fix 5: Clear OST File

1. Close Outlook
2. Navigate to AppData/Local/Microsoft/Outlook
3. Rename the .ost file (add .old)
4. Reopen Outlook - new OST created, full resync starts

Fix 6: Cached Mode Settings

• File, Account Settings, Change
• Enable "Use Cached Exchange Mode"
• Set slider to at least 12 months
• Use OWA to verify server-side emails exist

Fix 7: Calendar Sync

• Shared calendars: Remove and re-add from sharing invitation
• Room bookings: Permission issue on room resource mailbox
• Duplicates: Syncing same account via Exchange AND IMAP/CalDAV
• Google Calendar: Use ICS URL subscription for reliable sync

When to Escalate

• Multiple users affected (server-side)
• Autodiscover failing (DNS/Exchange config)
• Works on webmail but not desktop clients
• Started after migration or server update