Types of bounces

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).

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.

    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.
    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.

    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.
    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.
    Someone requested via e-mail to be included as a subscriber, but wrote to the bounce e-mail address.
    The receiver requested to be removed from any future message, but did not use the Unsubscribe link and wrote instead to the bounce address.
    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.

    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

    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.


Error code


Admin action (manually unsubscribed)0Unsubscribed by the administrator by changing the user status
Recipient used unsubscribe link2Unsubscribed by the user by clicking on the unsubscription link included in your message
Admin action (imported as unsubscribed)4Unsubscribed during the import process, by selecting the advanced option Import as unsubscribed (opted out)
Bounce8Automatic unsubscription due to incorrect email. More info
Due to spam complaint16Recipients clicking the "spam" button in the webmail client (e.g. Yahoo, Gmail etc.)
List-unsubscribe link32Recipients unsubscribed through a unsubscribe button in the webmail client (e.g. Yahoo, Gmail etc.)
List-unsubscribe mail64Recipients unsubscribed by sending an email to their ISP/webmail provider
Bounce (generic error)80Automatic unsubscription due to generic errors
Email was incorrect (bad syntax)81Email addresses with syntax errors (e.g. without a "@")
Email was invalid (bad domain)82Email related to a no longer existing domain
Emailwas incorrect (abandoned account)83Email address that was abandoned by the user. More info
Due to inactive recipient128Users who have not performed any activity in the latest period. More info
Due to blocked email256Email blocked. More info

Due to repeated bounces

512Email automatically unsubscribed due to repeated soft bounces. More info
Due to blocked email (target MTA block)2561Email blocked due to known spammer. More info
Due to blocked email (spam content)2562Email blocked due to spam content detected. More info
Due to blocked email (target MTA configuration error)2563Email blocked due to recipient configuration error. More info
Temporary opt-out due to repeated bounces5121Email temporarily unsubscribed due to repeated soft bounces. More info
Permanent opt-out due to repeated bounces5122Email permanently unsubscribed due to repeated soft bounces. More info