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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 it.

• HARD BOUNCE
  The message had PERMANENT errors during delivery. These typically are wrong email addresses. After one or more unsuccessful deliveries the message is removed from the send queue, and the corresponding subscriber is unsubscribed.
   
• SOFT BOUNCE – General
  This is a generic error, given when N:Bounce is unable to interpret the server's answer because it's not a standard or it is equivocal. Your mailing client cannot interpret formatted messages as MIME or DSN (see RFC from 2045 to 2049 for MIME general information and RFC from 1891 to 1894 for DSN specific information).
   
• SOFT BOUNCE – DNS Failure
  Problems with DNS. The message had PERMANENT delivery errors. It's an error given by the fact that the domain of the receiver (host) hasn't been solved in an equivalent valid IP. The cause could be temporary problems of the receiver's server, or that the destination domain doesn't exist or is written incorrectly.
  
  
• SOFT BOUNCE – Mailbox Full
  Mailbox is full. This message hasn't been delivered because the user's mailbox is temporarily exceeding its quota. These addresses should be removed from the Subscribers list only when the counter reaches a high number of errors compared with the number of deliveries.
   
• SOFT BOUNCE – Message Size Too Large
  The size of the message you sent is larger than the maximum size that addressee's mailbox allows. The error is due to the receiver, not to the sender. We advise you not to send messages bigger than 500 Kb, attachments included.
   
• GENERAL BOUNCE
  The message includes non-interpretable text parts. The message had PERMANENT delivery errors. After three delivery attempts, the message was removed from the send queue.
   
• MAIL BLOCK General
  The message includes non-interpretable text parts. The message had PERMANENT delivery errors. The receiver's mail server refused the message for an unspecified reason. See error details to have additional information.
   
• MAIL BLOCK Known Spammer
  The sender is recognised as a Spammer by the receiver's mail system. It is important to monitor these bounces. When they occur it is necessary to contact the recipients and try to understand why you are being flagged as a spammer.

• 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.
  
  
• TEMPORARY UNSUBSCRIPTIONS FOR REPEATED BOUNCES
  According to the type of bounces and the number of occurrences, the system will temporarily unsubscribe the recipient to avoid reputation's issues. The number of days might change depending on several factors including the number of unsubscriptions occurred in the past for the same reason
  
  
• DEFINITE UNSUBSCRIPTIONS  FOR REPEATED BOUNCES  
  After a number of temporary unsubscriptions (variable and depending on the severity of the bounce) the contact will be unsubscribed definitely  
  
  

When you decide to export unsubscribed, an error code for the unsubscription reason will be listed for each address. Please refer to the table below to match these codes with unsubscribe reasons.