Outlook for Mac (New Outlook) with IMAP account is very slow and often shows “Failed to download this message"

Question

I’m using New Outlook for Mac on macOS with an IMAP account that connects to my webmail (non‑Microsoft) provider.

Since around 1 April 2026 I’ve had serious performance and reliability issues:

• Messages I send sit in the Outbox for several minutes before they finally send.

When I open or reply to existing messages, Outlook often displays “Downloading attachments from server…” for a long time.

Quite a few messages show a popup “Failed to download this message” instead of loading their content.

The same mailbox works normally in webmail and has worked in Outlook for Mac in the past.

Troubleshooting I’ve already tried:

Checked Undo Send (set to 0 seconds).

Updated Office via Microsoft AutoUpdate to the latest build.

Fully quit and restarted Outlook and my Mac several times.

Used Reset account and also removed and re‑added the IMAP account multiple times.

Let Outlook fully re‑sync for over an hour after re‑adding.

None of this has fixed the problem. The issues only affect this IMAP account in New Outlook on my Mac; my other accounts are fine.

Questions:

Is this a known issue with IMAP accounts in New Outlook for Mac in recent builds?

Are there any advanced settings, logs, or patches I can apply to stop the Outbox delays and “Failed to download this message” errors?

If this is a bug, is there a recommended workaround (for example, switching back to Legacy Outlook, changing IMAP options, or disabling any “sync with Microsoft Cloud” features) until a fix is available?

Thank you for any guidance.

Answer 1

Hi Gurpreet Dhillon,

Welcome to Microsoft Q&A forum. 

At the moment, I’m not seeing an official article that specifically confirms this exact New Outlook for Mac + IMAP issue with long Outbox delays and “Failed to download this message". You can feel free to check out Fixes or workarounds for recent issues in Outlook for Mac - Microsoft Support if there's any updates about this.

For now, I would suggest reverting to Legacy Outlook to see if the issue still persists. And also you can kind check the IMAP account's advanced settings, such as: 

• SMTP authentication
• IMAP IDLE
• Sync interval
• Download headers only
• IMAP root folder.

For your reference: Advanced IMAP account settings for Outlook for Mac - Microsoft Support

If the problem only happens in New Outlook after those checks, the best next step is to report it from within Outlook using Help > Contact Support or Help > Feedback > Report a Problem. You can follow this article on how to do it: Give feedback about Outlook - Microsoft Support

I also recommend contacting Microsoft Support for additional insights through their live chat service. As forum moderators, we can only guide users to the appropriate support channels and provide troubleshooting steps based on our testing and official guidance from Microsoft.

Thank you for your understanding.

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