There are now two Outlook add-ins (COM and Web add-in). When should I use each one? 

There are two add-ins for Microsoft Outlook that can be used independently or in conjunction to provide optimal user experience and security coverage. Both add-ins are centrally managed and can be deployed to use the same policy configuration.

To understand when to use each add-in, see the Outlook Add-Ins - COM vs Web Add-In article. 

With all the different versions of Outlook, which one should I use?

This is entirely up to your organization. We support all versions of Outlook as we offer a COM and WEB add-in.

New Outlook is still in Preview only for Enterprise customers:
New Outlook for Windows: A Guide to Product Availability - Microsoft Community Hub

However, when using Classic Outlook on the Desktop, we recommend that you continue using the COM add-in, which is more performant and feature-rich than the Web Add-In for that platform.
Only the Web Add-In should be used for New Outlook, OWA, and Outlook for MACs.

How long will there be support for Classic Outlook and the COM Add-In?

Microsoft has confirmed that Support for Classic Outlook and COM will continue until 2029. We will also continue to support this until that time.
New Outlook for Windows: A Guide to Product Availability - Microsoft Community Hub.

When I send emails, I find that some of my emails are stuck in my Drafts folder.

Unfortunately, this is a known issue when you click away too quickly while the email is being processed. We have raised this with Microsoft to fix it. In the meantime, we have a workaround available for your users. For full details about the workaround, see the Web Add-In: On Send Feature article.

Does the Web Add-In currently support Large File Transfers?

Our Product Engineering teams are actively working on releasing this feature and it will be available in Q2 2024.

Do I need to switch to the New Outlook and the Web Add-In to use Microsoft Copilot?

Microsoft has announced support for Copilot in Classic Outlook in their recent roadmap updates Microsoft 365 Roadmap - See What's Coming | Microsoft 365. You can find further M365 requirements here Microsoft Copilot for Microsoft 365 requirements | Microsoft Learn.

Are there any prerequisites for using the new Web add-in? 

Customers wanting to use this must be native Microsoft 365 only, with no on-prem services.

The administrator must have access to the following:

• Office JS Manifest
• Administrative rights to the Office 365 admin portal
• Optional: O365 / Azure AD group to assign Office JS to a user group
• Outlook JavaScript API requirement set 1.8 or above
  Note: This add-in is not available for Exchange on-premise

Deployment is done via the Office365 Admin portal as an ‘Integrated App’. This will need to be performed as a one-off task by an Office 365 administrator with one of the below roles:

• Global Administrator
• Exchange Administrator combined with one of either Cloud App Admin, App Admin or Privileged Role Admin