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Can not send emails

Lou Boren 0 Reputation points
2026-05-04T13:07:28.2033333+00:00

I am able to receive emails - but for some reason can not send any emails - states not deliverable.

Outlook | Windows | Classic Outlook for Windows | For business

2 answers

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  1. Vergil-V 12,975 Reputation points Microsoft External Staff Moderator
    2026-05-04T13:50:18.9+00:00

    Hi @Lou Boren 

    Please note that this is a user-to-user support forum. Moderators, contributors including external Microsoft employees cannot directly intervene in Microsoft product features or access back-end systems. Our role is limited to providing technical guidance on reported issues, requests, or ideas.   

    From your description, the error indicating that you are not recognized as a valid sender is a common sign that the account has been flagged and temporarily placed under Restricted Entities for security reasons. 

    At this point, your IT administrator will need to review and remove the restriction. They can do this by visiting the Microsoft Defender portal at: https://security.microsoft.com, go to Email & collaboration > Review > Restricted entities or they can go directly using this link:  https://security.microsoft.com/restrictedentities. 

     

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    For additional guidance, your IT admin can refer to Microsoft’s documentation: Remove blocked users from the Restricted entities page - Microsoft Defender for Office 365 | Micros… 

    I hope this provides clarity and helps you move forward. If you have any updates or additional questions, please feel free to reach out. 

    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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  2. AI answer

    2026-05-04T13:07:53.99+00:00

    Use the non-delivery report (NDR) message to identify why messages are not being delivered, then apply the corresponding fix.

    1. Open one of the “not deliverable” messages and look for the error text (for example, “undeliverable”, “connection timed out”, “blocked”, etc.).
    2. Match the error type and apply the related action:
    • “Your message couldn’t be delivered” / invalid address
      • Check the recipient’s email address for typos.
      • Confirm the recipient is still using that address and that their domain name has not changed.
    • “Undeliverable”
      • This often indicates a problem on the receiving server or that delivery expired.
      • Verify the recipient address is valid.
      • Reduce the number of recipients and resend.
      • If using Outlook with another email app or connector, try sending directly from Outlook.com instead.
    • “Mailbox full”
      • Ask the recipient to free space in their mailbox and then resend.
    • “Can't connect to remote server/message expired” or “Connection timed out”
      • These errors usually resolve themselves.
      • If the NDR says the recipient’s provider will keep trying, no action is needed.
      • Otherwise, wait a short time and resend.
    • Errors mentioning “blocked”, “listed in”, or blacklist services (e.g., spamcop, spamhaus)
      • The recipient’s server has blocked the sending server as suspected spam.
      • Contact the support team for the recipient’s email provider and request that the account or domain be unblocked.
    1. If the account is Outlook.com-based and sending generally fails:
    • Ensure attachments are not larger than 25 MB.
    • If the account is new, be aware of temporary low sending quotas; limits increase as the account gains credibility. See “Sending limits in Outlook.com”.
    • If messages with a signature fail, try removing the signature and resending.
    1. If using classic Outlook for Windows and sending fails more broadly (not just to one recipient):
    • If the email account password was recently changed on the provider’s website, update the same password in Outlook.
    • Check antivirus software: some products block sending after detecting a virus; follow the antivirus help desk guidance.
    • Repair the Outlook installation if the client itself is corrupted.
    • Repair the Outlook profile or create a new one if the profile is damaged.
    • Start Outlook in safe mode (outlook.exe /safe). If sending works in safe mode, disable or remove problematic add-ins and/or create a new profile.

    Applying the specific fix that matches the NDR text usually restores the ability to send.

    References:

    AI-generated content may be incorrect. Read our transparency notes for more information.

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