Why are some of my outlook emails returned with an undeliverable notice from Microsoft?

Hi Donna, My name is Umar and I'd be happy to help you out with your question.

Please note: This is a user-to-user community forum. We are users just like you who help others. We are not employees of Microsoft.

If you are receiving "Undeliverable" notices for some of your emails in the Junk Mail folder in Outlook, it could be due to several reasons. Here are some steps you can take to troubleshoot and resolve the issue:

1. First, analyze the specific reason given in the undeliverable message. It might mention a temporary issue with the recipient's server, mailbox full, or exceeded spam limitations.
2. Double-check the email addresses for typos or misspellings, especially if you haven't used them recently.
3. Outlook's built-in junk mail filter may be incorrectly categorizing legitimate emails as junk. This can happen if the filter's sensitivity is set too high or if it is not properly trained to recognize certain senders or email patterns.
4. Sender Reputation: It is possible that the email addresses or domains of the recipients you are trying to email have been flagged as suspicious or have a poor sender reputation. This can happen if the recipients' email providers have strict spam filters in place. In such cases, the emails may be blocked or sent to the junk mail folder.

If your email address has been mistakenly flagged as spam by recipient servers, you can use online tools like Sender Score ([https://senderscore.org/]) to check your reputation and improve deliverability.

5. Email Content or Attachments: Certain types of email content or attachments can trigger spam filters and cause emails to be marked as junk or undeliverable. For example, emails with large attachments, suspicious links, or certain keywords may be flagged.

I hope this information is helpful. Please let me know if you have any other questions.

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Best Regards, Umar Majeed

I don't know if I'm replying on this thread or starting a new thread. My problem is related but different than Donna's. Starting around Jan 5, 2024, I suddenly got "Undeliverable" notices from attempts to send emails to certain recipients. Outlook App on Macbook Pro laptop and iPhone and website on the MacBook Pro. The notices come to my INbox after a day or two. Some of these are my replies in an active thread of exchanges, which eliminates any typos or similar causes - everything was fine until the problem appeared abruptly. The problem also appeared in many new emails. It does not matter if there are attachments or not. I've already spent much time unsuccessfully on the available Help avenues. It's extremely frustrating and I'd appreciate help!

I am having the same issue. I found this toggle in the Settings, under Mail and then Rules. It looks like every time I receive an email it is sending it to "@hotmail.com" as an attachment and since it is not a valid email it is responding to me and the sender as "undeliverable". I have turned it off and will see if that stops it.

Thank you Jill

I found rules in Vietnamese which I turned off and the problem went, I now see from a translation in the image above that it was causing the problem. I wasn't aware of rules before.

I was hacked a couple of months ago. I am not sure what the hacker was trying to achieve with the rules.

Thanks again

I am getting the following error messages when sending myself emails:

DU2PEPF00028D0B.mail.protection.outlook.com rejected your message to the following email addresses:

A communication failure occurred during the delivery of this message. Please try to resend the message later. If the problem continues, contact your email admin.

DU2PEPF00028D0B.mail.protection.outlook.com gave this error:Requested action not taken: mailbox unavailable (S2017062302).

Same here... sender cannot reach me with the authentication emails although I have added their domain to the safe sender list.