A bounce (aka bounceback) is the e-mail message you receive if a sent message doesn't reach its destination. A bounce typically indicates that the message has not been delivered. MailUp takes care of automatically logging and cataloging bounces for you. In certain cases, it also reacts to them by performing actions such as unsubscribing a recipient whose email address generated a Hard Bounce (see definition below).
For more information on how MailUp handles bounces, please see these articles in the MailUp Knowledge Base.
Here, let's look at how different types of bounces are defined. Note: these are pretty technical details. Unless you are interested in this topic, feel free to skip it 
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- MAIL BLOCK Spam Detected
The message has been classified as Spam by the receiver's mail system. These bounces are less serious than the previous ones, but you still need to monitor them for they are not an encouraging sign. Generally, they are issued based on automatic rules. You should contact the receiver to try to understand why your message was considered spam.
- MAIL BLOCK – Attachment Detected
The message has been refused by addressee's mail system because of a non-allowed attachment. It is best not to include attachments in your message and instead include a link to the file, which you can host on your Web site.
- MAIL BLOCK – Relay Denied
The message had PERMANENT delivery errors. The message has been blocked by the receiver's mail system because it refuses the connection to the sender's server. Generally this happens when the SMTP server cannot be detected or when accounts just can't receive any external mail. - AUTO REPLY
This is an automatic reply, usually due to the receiver's absence (e.g. Out Of Office, Vacation Message). NOTE: these messages do not always end up in the bounce mailbox; they can be sent directly to the sender's address, thus not being included in N:Bounce statistics.
- TRANSIENT BOUNCE
A temporary error: the delivery system found an error in the receiver's system, but it will try again. If the error persists, the system will give a Hard Bounce error.
- SUBSCRIBE REQUEST
Someone requested via e-mail to be included as a subscriber, but wrote to the bounce e-mail address.
- UNSUBSCRIBE REQUEST
The receiver requested to be removed from any future message, but did not use the Unsubscribe link and wrote instead to the bounce address.
- CHALLENGE RESPONSE
E-mail Challenge-Response systems were created to contrast the increase of spamming on mail servers. For example, systems like Challenge-Response, EarthLink SpamBlocker, and SpamArrest, Typically you will manually manage this kind of bounces, once per receiver. A Challenge-Response message is an automatic reply from the receiver meant to verify that the sender is a physical person and not just an automatic system. You will normally receive the message as a reply to your sender's address, and will have to click on the link included in the Challenge-Response mail to clear the way for the delivery.
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