Using the New Outlook for Mac for personal email and calendar
Hi,
I’m Dyari. Thanks for reaching out.
Since emails send normally in Legacy Outlook, in Outlook, select Tools and make sure it is set to Online.
Open Outbox and delete the oldest stuck message, especially if it has a large attachment.
Select Help > Check for Updates, install all updates, and restart Outlook.
Test by sending a simple email without an attachment.
If it still fails, what type of account are you using? Outlook.com, Microsoft 365, Gmail, or another provider? Do you see any error message?