Sending, receiving, and organizing email in Outlook.com
Sorry to hear that you are having trouble sending emails with attachments in the new Outlook. Since you mentioned that this happens only in the new Outlook, could you please provide a little more information?
Does the same email with the same attachment send successfully from Outlook on the web or classic Outlook? Does this happen with all attachments, or only with certain files or file types? Is the file stored locally on your computer, in OneDrive/SharePoint, or on a network drive? Is this a Microsoft 365 work/school account, an Outlook.com/Hotmail account, or an account from another email provider? Did this start recently, and if so, did anything change or update before the issue started?