Can someone use Outlook with a 3rd party email address without an M365 subscription?

Hi @TLRS, 

I hope you're having a great day. 
Thank you for reaching out, and I truly appreciate you taking the time to explain your situation. 

You can add multiple types of email accounts to Outlook at the same time, but you first need to sign in to the Outlook desktop app using your Work/School account that has a qualifying license (desktop app rights). Once Outlook is activated with that qualifying account, you can then add additional accounts (for example: Gmail, IMAP/POP custom-domain mailboxes, Exchange, etc.), as long as the email provider supports the appropriate connection method.

Qualifying licenses typically include: Microsoft 365 Business Standard/Business Premium; Microsoft 365/Office 365 Enterprise plans with desktop apps (e.g., E3/E5); Microsoft 365 Personal/Family. 

For further information, you can refer to these articles: 

• Add an email account to Outlook for Windows - Microsoft Support
• Licensing in new Outlook for Windows | Microsoft Learn 

I hope this information is helpful. Should you have any further questions or need additional assistance, feel free to reach out.   

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Yes, Outlook can be used with third‑party email addresses and with multiple domain mailboxes, as long as at least one qualifying Microsoft 365 license is present on the device.

Key points based on new Outlook for Windows licensing:

1. A qualifying license is required for the Outlook desktop app
   To use the Outlook for Windows desktop app (classic or new) with work/school accounts, a plan that includes desktop apps (for example, Microsoft 365 Business Standard, Business Premium, or similar enterprise/education plans) is required.
2. One licensed account unlocks Outlook on that device
   In new Outlook for Windows, when an account with a license that includes desktop apps is added (for example, Business Standard), that license applies to the Outlook app on that device. After that, additional accounts can be added regardless of whether those secondary accounts themselves include desktop app rights. This includes:
   • Other work/school accounts (for example, Business Basic or E1)
   • Personal Microsoft 365 Personal/Family accounts
3. Multiple domain mailboxes on multiple devices
   • On each device, as long as at least one account with a qualifying license is added to Outlook, multiple additional mailboxes from different domains can be configured in the same Outlook profile.
   • The same subscription can be installed on multiple computers according to its terms; once Outlook is licensed on each computer, multiple domain mailboxes can be added on each of those computers.
4. Third‑party email addresses
   As long as the third‑party accounts (for example, non‑Microsoft domains) are supported by Outlook protocols (IMAP/POP/Exchange/modern auth as applicable), they can be added as additional accounts after Outlook is licensed on that device.

So, if the subscription includes desktop apps and is installed/activated on up to the allowed number of computers, Outlook on each of those computers can host multiple domain email accounts at the same time.

References:

• Understand licensing in new Outlook for Windows
• Connected email accounts like Gmail and Yahoo not working in Outlook.com