How to Fix Email Sync Issues: 2026 Guide

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Introduction

Email sync issues happen when an email client stops showing the same mailbox as the server: messages appear on your phone but not on your laptop, sent emails are missing on other devices, deleted messages come back, or new mail arrives late. The cause is almost always one of six things: a device using POP3 instead of IMAP, a sign-in method the provider no longer accepts, wrong port settings or a blocked connection, a full mailbox, deletion settings that never expunge, or a client that does not save sent mail to the server.

Authentication is the most frequent cause today. Microsoft has disabled Basic Authentication for IMAP and POP in Exchange Online, and Google no longer lets Google Workspace accounts sign in to IMAP with a plain password from "less secure apps". A client that still signs in with a username and password fails with a generic "Authentication failed" or "Login failed" error. Both providers adjust their timelines, so confirm the current status in their official documentation, for example Microsoft's Basic Authentication deprecation page.

How do you find which cause you have?

Test the server from outside your device. Run an IMAP connection test with mailX using the same server, port and encryption as your client. If the test passes and your client still fails, the problem is local: account settings, the app, or its cache.

Check the account type and ports. IMAP should use port 993 with SSL/TLS. SMTP submission uses port 587 with STARTTLS or port 465 with SSL/TLS. A device set to POP3 or port 110 explains emails that vanish from other devices.

Check the sign-in method. Gmail, Google Workspace, Outlook.com and Microsoft 365 accounts should use OAuth 2.0 ("Sign in with Google" or "Sign in with Microsoft"), not a stored password.

Check storage and provider status. A full mailbox or a provider incident affects every device at the same time.

If you are not sure which server your domain uses, find your IMAP server settings before changing anything.

The Six Most Common Email Sync Issues

1. Emails missing on one device

Almost always caused by one device using POP3 while others use IMAP. POP3 downloads emails from the server and deletes them. IMAP keeps everything on the server and syncs across devices. If one device is configured with POP3, it pulls messages off the server before your other IMAP connected devices can see them.

Fix: Reconfigure the POP3 device to IMAP on port 993 with SSL/TLS enabled.

2. Authentication failed

How to diagnose: The client shows a generic "Login failed", "Authentication failed" or "Authentication required" message, even though the password is correct and works in webmail.

Gmail, Google Workspace, Outlook.com and Microsoft 365 expect OAuth 2.0 for IMAP connections. A client that authenticates with a stored username and password gets rejected without a clear explanation. Changing the password rarely helps: the client has to sign in again through the provider's OAuth consent screen. The cleanest way is to remove the account from the client and add it again with "Sign in with Google" or "Sign in with Microsoft".

Fix: Update your email client to a version that supports OAuth. Thunderbird, Apple Mail and recent Outlook versions handle it natively. For older clients, check whether your provider still offers app passwords as a fallback in the account's security settings.

3. Connection timeout

How to diagnose: Test whether the IMAP port is reachable from your network. Run the mailX IMAP test from the checklist above, or from a terminal run openssl s_client -connect imap.gmail.com:993 (replace the hostname with your provider's IMAP server). If the external test works but your device times out, a local firewall, proxy or VPN is blocking the port.

Your client can't reach the mail server. The three most common causes: wrong port number, firewall or VPN blocking the connection, or the server is down.

IMAP uses port 993 (encrypted). SMTP uses port 587 (submission with auth). If your client is configured for port 110 (POP3) or port 25 (SMTP relay), it's using the wrong protocol or port for what it's trying to do. Corporate firewalls and some VPNs block non-standard ports.

Fix: Verify your proxy/firewall settings allow ports 993 and 587.

4. Mailbox quota exceeded

How to Diagnose: Check if you can send but not receive, or if the client reports "Storage full."

When your mailbox hits its storage limit, the server stops accepting new mail and may reject sync requests. Gmail provides 15GB shared across Gmail, Drive, and Photos on free accounts. Google Workspace accounts vary by plan.

Fix: Delete old emails, empty trash and spam folders, and remove messages with large attachments. Search for "has:attachment larger:10MB" in Gmail to find the biggest storage consumers.

5. Deleted emails keep reappearing

How to Diagnose: Observe if deleted items return after a specific time interval.

IMAP marks emails as "\Deleted" but doesn't remove them until an EXPUNGE command is issued. Some clients do this immediately; others wait. If your client isn't expunging properly, deleted emails reappear on the next sync.

Fix: Check your client's IMAP deletion settings. Look for options like "Move to Trash" vs "Mark as Deleted" and "Expunge on exit" vs "Expunge immediately."

6. Sent emails don't appear on other devices

How to Diagnose: Verify that the "Sent" folder is checked in the server's folder list.

Your client sends via SMTP but doesn't save a copy to the IMAP Sent folder. The email leaves your outbox successfully, but no other device can see it because the server's Sent folder never received a copy.

Fix: Enable "Store sent messages on server" or "Save copy to Sent folder" in your client settings. Most modern clients do this automatically, but older or misconfigured clients skip the step.

How do you fix sync problems on each device?

Once the server side checks out, the remaining causes are local. These are the standard resets for each platform. Menu names change between versions, so follow the closest equivalent on your device.

iPhone and iPad (Mail app)

Check that the account is IMAP or Exchange, not POP, in the account's settings in Settings.

Turn Mail off and on again for that account. If folders are still missing or messages loop, delete the account from the device and add it again. Your mail stays on the server and downloads again.

Under Fetch New Data, choose Push if your provider supports it, or a short Fetch interval.

Windows (Outlook)

Close Outlook and run the Inbox Repair Tool (SCANPST.EXE) on the local data file.

If sync still fails, create a new Outlook profile from Control Panel > Mail > Show Profiles and add the account again.

For Microsoft 365 accounts, make sure Outlook is up to date and signed in with modern authentication.

Android (Gmail app)

Clear the Gmail app's cache in Settings > Apps > Gmail > Storage. Clearing local data does not delete emails from the server.

Reopen the app and sign in again if asked.

Check that account sync is turned on for that account in the system settings.

Mac (Apple Mail)

Select the affected mailbox and choose Mailbox > Rebuild. Apple Mail downloads the messages again from the server.

If the account keeps failing, remove it in System Settings > Internet Accounts and add it again.

Check the account's server settings in Mail settings > Accounts > Server Settings and compare them with your provider's IMAP and SMTP values.

The Connection to Deliverability

IMAP sync issues don't directly affect your ability to send email. But they affect your ability to see what's happening with your email. If replies aren't syncing, if bounces aren't appearing, if spam complaints are going to a device that's not syncing properly, you're flying blind.

More importantly, IMAP is where engagement signals originate. When someone opens your email, replies to it, or moves it from spam to inbox, those interactions flow through IMAP back to the server. Inbox providers use that engagement data to evaluate your sender reputation.

Mailwarm's warmup generates these IMAP-level engagement signals from 50,000+ real inboxes. Real opens, real replies, real positive interactions that providers count when determining whether your next campaign reaches the inbox.

Frequently asked questions about email sync issues

Why does my email client show connected but new emails aren't appearing?

The client may be connected but not actively checking for new mail. Enable IMAP IDLE (push) if your client supports it. Without IDLE, the client checks the server at a fixed interval, often several minutes, which can look like a sync failure while you wait for a specific message. Check the fetch or refresh interval in the account settings.

Should I switch to a web client if sync keeps failing?

Webmail such as Gmail in the browser or Outlook on the web reads the mailbox directly on the provider's servers, without IMAP. If IMAP keeps causing problems and you don't need a desktop client, webmail is the most reliable fallback. It is also a quick test: if a message shows in webmail but not in your client, the problem is the client.

Can sync issues cause me to miss important emails?

Yes. If your client can't connect, you won't see new messages. If your mailbox is full, new emails can bounce back to the sender. If a POP3 device downloads and deletes messages before your IMAP devices sync, those messages disappear from your other devices.

How do I check if my client supports OAuth 2.0?

Look in the client's account setup for "OAuth", "Modern Authentication", or "Sign in with Google" and "Sign in with Microsoft". If none of these options exist, the client only supports password sign-in and will not work with Gmail, Google Workspace or Microsoft 365 accounts that require OAuth. Update it or switch clients.

How do I confirm the problem is not on the server?

Test the server independently of your device. A passing IMAP connection test with the same hostname, port and encryption means the server accepts connections, so the issue is in the client settings, the app or the local cache.

Before you reset devices or reinstall apps, run a free IMAP connection test with mailX. It tests the connection to your IMAP server in seconds, with no signup.

Written by Daniel Nwankwo

Community and Content manager