And when you open up those emails you will have something like this:
Sample 1:
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.
The following address failed:
“email address”
For the following reason:
multiple delivery attempts failed
…
Or
Sample 2:
Your message to “email address” couldn’t be delivered.
“email address” wasn’t found at “domain name”
Unknown To address
How to Fix It
The address may be misspelled or may not exist. Try one or more of the following:
Send the message again following these steps: In Outlook, open this non-delivery report (NDR) and choose Send Again from the Report ribbon. In Outlook on the web, select this NDR, then select the link “To send this message again, click here.” Then delete and retype the entire recipient address. If prompted with an Auto-Complete List suggestion don’t select it. After typing the complete address, click Send.
Contact the recipient (by phone, for example) to check that the address exists and is correct.
The recipient may have set up email forwarding to an incorrect address. Ask them to check that any forwarding they’ve set up is working correctly.
Clear the recipient Auto-Complete List in Outlook or Outlook on the web by following the steps in this article: Fix email delivery issues for error code 5.1.10 in Office 365, and then send the message again. Retype the entire recipient address before selecting Send.
If the problem continues, forward this message to your email admin. If you’re an email admin, refer to the More Info for Email Admins section below.
….
What Has Happened?
Simple answer: Your email address has either been spoofed or your email account has been compromised.
The failed delivery email above happens if the recipient does not exist or if your email server is already marked you as a spammer and therefore all emails from your address (domain) will be rejected.
If your email server (that sends out your emails) is marked as a spammer, then probably you will see tons of Failed Delivery emails in your inbox as well.
Email spoofing is the process of sending emails messages with a forged sender address.
If someone has stolen your email address and is using it in their spoofing attacks, you will probably have some undelivered email notifications in your inbox.
A spammer can send thousands or millions of emails delivering phishing emails to email addresses unknown to you!
This is a strategy spammers are using to send millions of spam and phishing emails with a forged sender address (other people’s email address) in hope that some of them actually get through and someone clicks on the attachment or the link in their emails.
This is the easiest way an attacker can infect other people’s computers and networks and inject their ransomware. They can then encrypt victim’s data and ask for money in form of cryptocurrencies to deliver the decryption key to the victim.
This process will not cost the attacker anything and most probably is automated.
If spammers or attackers have somehow found your email address and your email password, they can send emails as YOU using your email server and email address.
The recipient email server has no way to identify that this is not the owner of the account! Like the example in email spoofing, they will use your email account to send out millions of spam and phishing emails.
Solving these issues is frustrating, takes time and in some cases requires special technical skills. However, you can follow these simple best practices to fix these issues in some degree:
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