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E-mails sent from Outlook classic don't send unless New Outlook is switched back to classic

Maxime Joannette 20 Reputation points
2025-12-03T21:40:32.0833333+00:00

I have some users having trouble with the New/Classic Outlook situation.

They're basically locked into using the New Outlook due to the newest features available (don't have the exact detail on this), but I had them set Outlook (classic) as their default mail program for the mailto protocol, since our application uses the Windows Mail API to open the Send e-mail form, and this doesn't work with the New Outlook.

It apparently seems to work fine, until they figure out none of the e-mails are getting sent, and they're stuck in their Outbox unless they switch to classic Outlook .

Can you help figure out which server or client settings could be causing this?

Thanks

Outlook | Windows | Classic Outlook for Windows | For business
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  1. Kha-N 5,455 Reputation points Microsoft External Staff Moderator
    2025-12-04T00:56:47.3266667+00:00

    Hi @MaximeJoannette,

    Welcome to Microsoft Q&A, and thank you very much for reaching out to us.

    Please note that our forum is a public platform, and we will modify your question to hide your personal information in the description. Kindly ensure that you hide any personal or organizational information the next time you post an error or other details to protect personal data.

    Please note that as a Microsoft Q&A moderator, I don’t have access to your specific configuration, and my testing environment is limited. I can only assist based on available documentation and resources, but I’ll do my best to help.

    To assist you better, when you say “some users,” do you mean the issue only affects few users while others with the same setup can send without any problems?

    Based on your description, from my research, New Outlook does not support legacy MAPI-based automation according to this Microsoft article here.

    User's image

    As the sending process depends on client-side MAPI automation, which means Classic Outlook needs to be active for emails to go out. When users switch to New Outlook, that dependency could breaks, result in messages to get stuck in the Outbox.

    If you prefer to keep using MAPI-based automation, you can run Classic Outlook and New Outlook side by side by launching them through the Windows search bar as a workaround.

    User's image

    For more details, you can check this Microsoft article here.

    If you want to fully transition to New Outlook, you can consider registering your application in Microsoft Entra and migrating to Microsoft Graph as an alternative. Graph allows sending emails directly from a mailbox without relying on the Outlook client. For detailed guidance on using this Graph capability, you can check this Microsoft Article here.

    I hope this helps.

    If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".     

    Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

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  1. Q&A Assist
    2025-12-03T21:40:44.8533333+00:00
    AI generated content. This question contains an answer created with AI from Q&A Assist. Learn more

    The issue you're experiencing with emails getting stuck in the Outbox when using the New Outlook could be related to how Outlook interacts with the mail protocols and settings when switching between the New and Classic versions. Here are some potential causes and solutions:

    1. Default Mail Client: Ensure that Outlook is set as the default mail client in Windows. If it is not, it may cause issues with sending emails through the mailto protocol. You can check this by searching for "Default Apps" in Windows settings and confirming that Outlook is selected as the default for email.
    2. MAPI Configuration: Since your application uses the Windows Mail API, it may not fully support the New Outlook. MAPI (Messaging Application Programming Interface) only works with the 32-bit version of Outlook, and if the New Outlook is being used, it might not handle sending emails correctly. Ensure that users are using the Classic Outlook version if MAPI is required.
    3. Delegate Sent Items Style: If emails are being sent from a shared mailbox and remain in the Outbox, check if the DelegateSentItemsStyle registry value is set correctly. This can affect how sent items are handled and may require manual Send/Receive operations to clear the Outbox.
    4. Connection Issues: If users are on a metered connection, Outlook may restrict sending emails. Ensure that they are connected to a network that allows full access to email services.
    5. Add-ins and Extensions: Sometimes, third-party add-ins can interfere with email sending. It may be beneficial to compare the add-in list on the affected clients with a clean installation to identify any problematic add-ins.

    If these suggestions do not resolve the issue, further investigation into specific server settings or user profiles may be necessary.

    References:

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