Gmail slow to sync in Outlook

Question

Further to my earlier question today about this, in my own efforts I have found another twist to this trail...

If I use "New" Outlook it all works OK, it is when I switch to "Classic" Outlook it all stops working.

So what is classic outlook trying to do and how do I make it work?

And I do not find the functionality of "new" outlook any good. It is a backward step.

One example - get a red squiggly line to indicate a spelling error:-

Old Outlook right click for a spelling correction and you make your choice

New Outlook right click for options, right click again for spelling to find your options - WHY?

New Outlook has been thrust upon us, with all the most useful options buried away somewhere else.

If it ain't broke don't try to fix it.

Answer 1

Classic Outlook and New Outlook use different connection and authentication methods for Gmail. That is why Gmail sync works in New Outlook but is slow or failing in Classic Outlook.

For Classic Outlook, there are two common Gmail‑related issues in the provided information:

1. Classic Outlook not syncing messages/folders correctly
   • Classic Outlook must use the correct account type and protocol. For Gmail, that is an IMAP account, not Exchange or Microsoft 365.
   • If Classic Outlook is not syncing existing messages and folders, verify that the account is configured as an IMAP Gmail account, not as another type. If the account type is wrong, remove the account from Classic Outlook and add it again as a Gmail/IMAP account.
2. Problems adding or signing in to Gmail in Classic Outlook
   • When adding Gmail to Classic Outlook, Outlook now uses a browser sign‑in flow. If IMAP is not enabled in Gmail, or if the Google sign‑in flow is blocked, Classic Outlook will not sync correctly.
   • If the error “We weren’t able to set up your Google account with IMAP. You may need to make changes to your Gmail settings.” appears, IMAP may be disabled on the Gmail side. IMAP must be enabled in Gmail settings as described in Google’s “Add Gmail to another email client” guidance.
   • There is also a known issue where Gmail sign‑in in Classic Outlook returned “The browser or app may not be secure.” or similar IMAP setup errors. This has been fixed in newer Outlook builds by changing the sign‑in to open another browser window. Ensuring Classic Outlook is updated to a build that includes this fix is required for reliable Gmail connectivity.

In summary, Classic Outlook is using the traditional IMAP account model and a browser‑based OAuth sign‑in for Gmail. To make it work reliably:

• Confirm the Gmail account is added as IMAP in Classic Outlook (not Exchange/Microsoft 365).
• Ensure IMAP is enabled in the Gmail account settings.
• Update Microsoft 365 Apps so Classic Outlook includes the latest Gmail sign‑in fixes.

The usability differences between New and Classic Outlook (such as extra clicks for spell‑check corrections) are by design changes in New Outlook’s interface; Classic Outlook remains the version that supports the traditional, more feature‑rich desktop experience and is the recommended one when features like automation or advanced add‑ins are needed.

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References:

• Troubleshoot Outlook email setup
• Unable to sign in or add a Gmail account to classic Outlook

Answer 2

Hi Alan Buxton,

Welcome to Microsoft Q&A forum. 

To help narrow this down quickly, could you please share:

• Classic Outlook version/build (File > Office Account > About Outlook) and your Windows version
• Is this a personal Gmail or Google Workspace account?
• In Classic Outlook, is the account added as IMAP? (File > Account Settings > Account Settings > Type)
• When it stops syncing, do you see any Send/Receive status or code (for example 0x800CCC0E / 0x800CCC0F), or does it simply stop updating without prompting you to sign in again?

Once you send those details, I will suggest the most suitable next steps for Classic Outlook.

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