GoKawiilMicrosoft is investigating a known issue that prevents some Classic Outlook users from sending emails via Outlook.com.
Affected users are being warned that their message hasn't reached some intended recipients, and they will encounter this problem more often when the Outlook.com account they use to send email is an Outlook profile linked to another Exchange account.
"This message could not be sent. Try sending the message again later or contact your network administrator," the non-delivery report (NDR) error displayed when sending or replying to emails reads.
"You do not have the permission to send the message on behalf of the specified user. Error is [0x80070005-0x0004dc-0x000524]."
Microsoft added that another condition that may trigger these errors is that the sender's account has an Exchange Online mail contact with the same SMTP address.
Classic Outlook non-delivery report (NDR) error (Microsoft)
While investigating this issue and still looking for a fix, the Outlook team shared several workarounds that may help affected customers temporarily mitigate the issue.
Microsoft recommends removing the M365 account Address Book so that the Outlook client does not check it when sending emails, hiding the Outlook.com contact from the Microsoft 365 account Global Address List (GAL).
Other alternatives include creating a new classic Outlook profile that includes only the account receiving NDR errors, and using the New Outlook client or Outlook.com on the web to send email from the affected account.
Over the last two weeks, Microsoft fixed two other known issues, including one that caused Classic Outlook to crash when enabling the Microsoft Teams Meeting Add-in and another that triggered 0x800CCC0F and 0x80070057 errors when synchronizing Gmail and Yahoo accounts.
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