Unable to send email through Outlook using MAPI

While the "New" Outlook added a number of new features and capabilities, it also removed the ability to handle sending email from 3rd party applications such as Atrex.  This is confirmed via the following feature comparison via Microsoft support: New Outlook vs Classic

If you must have the ability to send email from Atrex through Outlook using MAPI, you will need to move back to Classic Outlook.  Instructions from Microsoft on how to do this is available at: Switching to Outlook Classic

If you want to keep using the New Outlook, Atrex 23 and higher provides the ability to send send directly through the Microsoft 365 API.