That message you receive when an email doesn't make it to its destination identifies a bounced email. The term bouncing email describes when the receiving side server of an email message transaction rejects or never receives a sent email for any reason and the message is sent back to the sender. In physical mail terms, it's the same thing as receiving a letter that's been returned to sender.
Non-delivery report is the technical term for the message you receive when you bounce an email. Receiving a NDR simply means something went wrong and the person you sent an email to never received it. A NDR may contain an error code that identifies what what wrong in the email sending process. These codes are different depending on the email service.
A hard bounce means that the email can't be delivered at all. Hard bounces occur when you send an email to an invalid address, when the recipient blocks your address, and when the receiving server has blocked the sending server. The email message will return bounced if the address it's sent do doesn't exist. The reason the address is invalid can be as simple as a typographical error. However, the address you're sending to may have been deleted or changed without enabling forwarding to the new address.
Soft bounces occur when the email reaches the recipient's address and gets rejected. The reason for a soft bounce can be because the receiving server is overloaded, the receiver's email account is full, or the message is larger than the server's maximum acceptable size. Soft bounces may include the reason the message was rejected in the NDR so you can adjust the email to fix the problem and resend it.
Email messages usually go to and from servers without a problem, but they're not impervious to connection problems. For a message to send completely, the sending and receiving servers need to establish a connection and maintain it long enough to transfer the message. The email won't send correctly if that connection is interrupted and you may receive a bounce email. Emails that have larger file attachments are more prone to bouncing because of a bad connection.
A bounce email that shows up in your Inbox might not be real. It's possible to generate fake bounce emails through a custom Web page or custom email service. Additionally, email services like Incredimail let you send a fake bounce email as a reply.
Problems Indicated by Bounces
Receiving dozens of bounced emails from messages you never sent is indication of some sort of email account compromise. This could mean that either someone is sending spam email with your account or using your account as the return address. The former can be fixed by changing your password and password reminder questions. The latter can somewhat be resolved by blocking the addresses that are sending the NDRs, but you'll lose the ability to receive legitimate ones from those servers.
- PCMag.com: Definition of Bounced E-mail
- PCWorld: 100 E-mail Bouncebacks? You've Been Backscattered.
- Microsoft: Non-Delivery Reports
- IncrediMail: IncrediMail Help Page
- Mail Chimp: What's the Difference Between Hard and Soft Bounces?
- Campaign Monitor: What Is a Hard and Soft Bounce, and How Do I Reduce the Number of Emails That Bounce?
- HubSpot: 19 Email Deliverability Terms Every Marketer Should Know