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

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.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.