In computer jargon, a bounced e-mail is one that never arrives in the recipient's inbox and is sent back, or bounced back, to the sender with an error message that indicates to the sender that the e-mail was never successfully transmitted. But what happens when someone sends an e-mail out into cyberspace, and why do e-mails sometimes bounce back?
When a user attempts to send an e-mail, he is telling his e-mail system to look for the domain of the recipient (for example, webopedia.com) and the domain's mail server. Once the e-mail system makes contact with the recipient's mail server, the mail server looks at the message to determine if it will let the message pass through the server. If the recipient's server has predetermined that it is not accepting e-mails from the sender's address (for example, if it has blocked the address for anti-spamming purposes), the server will reject the message and it will subsequently bounce back to the sender. The message will also bounce back to the server if the mail server on the recipient's end is busy and cannot handle the request at that time. When an e-mail is returned to the sender without being accepted by the recipient's mail server, this is called a hard bounce.
Once the e-mail has been accepted by the recipient's mail server there are still ways for the message to be rejected. The mail server has to determine if the recipient (for example, email@example.com) actually exists within its system and if that recipient is allowed to accept e-mails. If the recipient's address does not exist on the mail server, then the message will be rejected because there is no one to deliver the message to. If the sender misspells the recipient's address (for example, firstname.lastname@example.org) then the system will recognize this as a nonexistent address and bounce the message back. If the recipient exists but does not have enough disk space to accept the message (i.e., if his e-mail application is filled to storage capacity) then the message will bounce back to the sender. Some mail systems predetermine a maximum message size that it will accept and will automatically bounce the message if it exceeds that size and some mail systems predetermine a maximum amount of disk space the user is allowed to occupy on the server. When an e-mail is returned to the sender after it has already been accepted by the recipient's mail server, this is called a soft bounce. Some mail servers are programmed to accept incoming e-mails and store them for further analysis without initially checking to determine if the recipient exists or is even capable of receiving the message.
Occasionally, a network failure at the sender or recipient end will cause an e-mail to bounce back to the sender. Typically, a bounced e-mail returns to the sender with an explanation of why the message bounced.