Properly authenticated messages received from postmaster@outlook.com, postmaster@microsoft.com, or service.microsoft.com are legitimate messages that might be sent to your work or school account, or your personal Microsoft account where Microsoft is the email provider*. (Be sure to validate that the email is in fact from the postmaster and not a spoofed email. See Phishing and suspicious behavior in Outlook.)
Note: *Accounts where Microsoft is the provider include personal Microsoft accounts such as Outlook.com, Hotmail.com, Live.com, or MSN.com; or work or school accounts you use with Microsoft 365 or Exchange Online.
A response from the from postmaster@outlook.com, postmaster@microsoft.com, or service.microsoft.com usually means the message you tried to send couldn’t be delivered to one or all the intended recipients.
How to fix the issue and resend the message
Review the postmaster message for possible reasons why your message was undeliverable. To do this try scrolling down in the message and look for a section that might be named something like, "Diagnostic information for administrators." This section usually includes the reason why the message couldn’t be delivered.
Examples of why an email was undeliverable include entering a recipient’s name incorrectly or entering the name of someone that no longer found in the organization's address book. Or perhaps you tried to attach a file larger than 10MB.
Depending on the reason for your message to be undeliverable, try the following:
Verify recipient’s email address and resend your message
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In the new copy of your message, select the recipient's email address in the To box and then press the Delete key.
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Remove the recipient's email address from the Auto-Complete list (a bad or outdated entry could be causing the problem):
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In the To box, start typing the recipient's email address until it appears in the Auto-Complete drop-down list.
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Use the Down Arrow key to select the recipient from the Auto-Complete drop-down list and then select Delete.
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In the To box, continue typing the entire recipient email address. Be sure to spell the address correctly.
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Select Send.
Reduce the size of your attachment
If your original message had an attachment larger than 10 MB, the Send Again option might not be available or might not work. Instead, resend the message from your Sent Items folder. For more information, see Resend an email message.
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Try reducing the size of the attachment. See Reduce the size of pictures and attachments in Outlook email messages.
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Or if the file is still too large, save the file to a cloud service such as OneDrive and share a link. See Send large files with Outlook.
Ask the recipient to check for broken forwarding rules or settings
If everything looks correct, as the recipient if they have an email rule that forwards incoming emails message from you to another destination. Their rule could have tried to send a copy of your message to a bad email address.
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