sender as a 'spam' sender, or to blacklist a certain sender. Moreover, a temporary suspension of the recipient's email or a temporary error on the server are also causes of a soft bounce. Delivery errors Errors may occur at multiple places in mail delivery. A sender may sometimes receive a bounce message from their own mail server, reporting that it has been unable to send a message, or alternatively from a recipient's mail server reporting that although it had accepted the message, it is unable to deliver it to the specified user. When a server accepts a message for delivery, it is also accepting the responsibility to deliver a bounce message in the event that delivery fails. Bounce due to lack of disk space When an e-mail arrives at the destination server for an address (such as mymail.example, when sending to alice@mymail.example), it may be that the mail daemon is unable to deposit the message in the specified user's mailbox if the underlying hard drive of the server has insufficient space. Bounce due to unreachable destination When sending an e-mail, the service from which the e-mail is sent may be unable to reach the destination address. In such case, the sender would receive a bounce message from their own mail server. Common causes for mail servers being unable to reach a destination: Unable to resolve the destination address. For example, if the domain name does not exist. Unable to establish a connection with the destination address. For example, if the IP address is not assigned to a server, or if the server is offline. Bounce from forged message Users may receive erroneous bounce messages about messages they never actually sent. This can happen in particular in the context of email spam or email viruses, where a spammer (sender) may forge a message to another user (intended recipient of spam), and forges the message to appear from yet another user (a third party). If the message cannot be delivered to the intended recipient, then the bounce message would be "returned" to the third party instead of the spammer. This is called backscatter. Other causes Had the library.example mail server known that the message would be undeliverable (for instance, if Jill had no user account there) then it would not have accepted the message in the first place, and therefore would not have sent the bounce. Instead, it would have rejected the