A guide to bounces

If any email that we send fails to be delivered, we receive an error report from the destination server. Such reports are automatically processed by Sign-Up.to, and are always split into two categories: soft and hard bounces.

Soft bounces

The majority of error reports we receive are interpreted as soft bounces. These are messages informing us of possible network errors, destination server mis-configuration or unavailability, or recipient mailbox quota issues preventing successful email delivery. Essentially this means that the issue with delivery is temporary, so for now the system just makes a record of this address bouncing the email.

A count of soft bounces for each recipient is maintained. Should this value reach three across any 15 day or greater period, we mark the email address as suspended. The 15 day period between the first and final consecutive soft bounce is required to prevent temporary issues from triggering suspension on a regular use address. Should a client suffer poor email reputation Sign-Up.to will reduce the soft bounce count required before suspending an address in order to protect against further reputation degradation.

Hard bounces

If we receive an error report informing us that the email recipient or domain is invalid, we have received a hard bounce. It implies that there would be no benefit in retrying or waiting before attempting delivery again. At this point we mark the email address as suspended.

SMS bounces

Sign-Up.to provides delivery report information on SMS messages too. Mobile delivery reports are currently less involved than email, so depending on the mobile network in question, we either get a positive 'delivered', a negative 'failed', or in the worst case scenario, no report at all. We treat all SMS 'failed' reports in the same way that we treat soft email bounces which makes allowances for flat batteries and holidays.

Suspended addresses

Having suspended addresses within your account does you no harm at all. The system won't send to these addresses once they're suspended, and if an address has been suspended for your whole billing cycle, you won't be charged for it in that period. Furthermore, if the owner of that address performs any action that suggests that address is once again in use (for instance if they open an old campaign or click a link), we'll immediately re-activate that address so you can send to it once again.

The suspended subscribers list (found under the Collect tab) is made up from campaigns that have bounced back from specific email addresses. If the address is not recognised or not in use any more, the campaign will come back as a hard bounce and will immediately be added to this suspended subscriber list. Any campaigns you send will not be sent to those on the suspended subscribers list. Soft bounces which are caused due to possibly temporary events, such as network issues or full inboxes etc., will become hard bounces if they are rejected three times in the period of 15 days or longer. These again will be added to the suspended subscribers list.

Bounce codes

Sign-Up.to interpret email bounce codes in accordance with the RFC 3463 – Enhanced Mail System Status Codes Memo. While most email servers follow the same guidelines, some do differ, so explanations provided in this document and within Sign-Up.to applications should be considered an indication rather than an absolute truth.

Some email servers also send a brief explanation of the error along with the error code. Where this occurs, Sign-Up.to will show this additional information in the application. Due to the difference in interpretation of the guidelines, this information may not always match up with this documentation.

Codes are formed of three digits separated by decimal points. In the case of bounce errors, the first digit of these codes is either a four or a five. A four typically means a persistent but not necessarily permanent error, whereas five is usually a permanent error. In the explanations below, Sign-Up.to treat errors beginning with four as soft and five as hard, except where stated otherwise.

  • 4.0.0 / 5.0.0. Undefined persistent error. Unfortunately some systems only issue general warnings such as this. As these error messages are vague, we treat them as soft bounces.
  • 4.1.x / 5.1.x. Destination address error, usually caused by a bad address.
  • 4.2.x / 5.2.x. Destination mailbox error, usually caused by a quota limit or other rule/error on the recipient's server. Sign-Up.to treat all these errors as soft bounces.
  • 4.3.x / 5.3.x. Destination mail system error or misconfiguration.
  • 4.4.x / 5.4.x. Networking issues with intermediate or destination mail server.
  • 4.5.x / 5.5.x. Mail delivery protocol status. This can be caused by any number of factors, from delivery mail server protocol errors to intermediate network issues or delays.
  • 4.6.x / 5.6.x. Unsupported media or message content error.
  • 4.7.x / 5.7.x. Security or policy status has triggered an error. It's possible that an aggressive anti-SPAM policy or undesirable previous content has caused the system to reject mail. Sign-Up.to treat these as hard bounces so as not to further aggravate the destination mail server.


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