why message couldn't be delivered

The error message you received indicates that the sending email address was not recognized as a valid sender. This can happen for several reasons, with the most common being that the email address is suspected of sending spam. In such cases, the email server may block the address to protect users from potential spam or malicious content.

To resolve this issue, it is advisable to contact your organization's email administrator. They can investigate the situation further and determine if the email address can be reinstated as a valid sender. Providing them with the specific error message you received will help in diagnosing the problem more effectively.

References:

• Email non-delivery reports and SMTP errors in Exchange Online

Dear @service,

Thank you for reaching out to the Microsoft Q&A forum. We are happy to assist you.

First, please try sending an email using Outlook on the web to determine whether the issue occurs only in the desktop app or across both platforms.

Next, contact your IT administrator and ask them to review the following steps:

Check for quarantined or flagged messages in the tenant:

1. Sign in to the Microsoft 365 Defender portal with an admin account.
2. In the left navigation, select Email & Collaboration > Review > Quarantine.
3. Use filters to search for the affected user’s email address. If messages are quarantined:
   1. Review the reason (e.g., spam, malware).
   2. Select the message and choose Release message if appropriate.

Or you can remove blocked users from the Restricted Entities page by follow the instructions provided in this link: Remove blocked users from the Restricted entities page

Verify DNS Records: If you're sending from a custom domain, please ensure that SPF, DKIM, and DMARC records are correctly configured. Misconfigured records can cause your emails to be rejected by recipient servers.

Visit here for more details: Set up DKIM to sign mail from your cloud domain

Recent Sending Activity: If there have been recent bulk email campaigns or messages containing links or attachments, these may have triggered spam filters. We recommend reviewing the content and formatting of those emails to ensure compliance with best practices.

Note: If you’re an end-user, please coordinate with your Office 365 Global Administrator to handle these steps.     

I hope this information is helpful. If you need any further assistance, please feel free to reach out. We are here to help. Thank you very much for your understanding and cooperation.

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