Overview


When an email is unable to be delivered, typically the recipient's mail server will return a bounce. Within that bounce are often two things: a simple three digit code and a somewhat human-readable message. Together, these are used to categorize the bounce message and to provide more information about why the message could not be delivered. Listed below are some of the common codes that you may see while examining your bounce reporting.


Keep in mind that every mail server is unique and there are no hard and fast rules governing bounce responses. The bounce codes displayed with Engage reporting are the codes that our servers receive from the receiving mail servers.   It is important to know that it’s possible that these mail servers may provide codes that do not really reflect the true nature of the bounce. Questline continually works to evolve its bounce management processes and bounce classifications to be as effective as possible.


Temporary Failures


A 400-level message is usually returned when some sort of transient error is encountered during the message transaction. These types of responses are usually not a cause for alarm as most of these will iron themselves out given a little time.


In general, when Engage receive a 400-level bounce message, the system will retry sending that message for up to 72 hours. If the message continues to bounce throughout that timeframe, the system will eventually classify the message as a Block bounce and stop sending. Addresses that Block bounce are not suppressed from future mailings and will continue to be sent future email.


NOTE: Temporary failures are not recorded in Engage reporting, as they may succeed upon retry or will eventually come back with a Block or Hard bounce.



421


Messages are temporarily deferred because of recipient server policy reasons. Usually because of too many messages or connections in too short a timeframe.


Example


421 4.7.0 [GL01] Message from (X.X.X.X) temporarily deferred421 4.7.1 : (DYN:T1) http://postmaster.info.aol.com/errors/421dynt1.html (throttled)421 4.7.0 [GL01] Message from (X.X.X.X) temporarily deferred - 4.16.50. Please refer to http://postmaster.yahoo.com/errors/postmaster-21.html


What to do with this response


We’ll continue to retry deferred messages for up to 72 hours for a response like this.




450


The message failed because the user's mailbox was unavailable, perhaps because it was locked or was not able to be routed at the time.


Example


450 4.2.1 The user you are trying to contact is receiving mail too quickly. Please resend your message at a later time. If the user is able to receive mail at that time, your message will be delivered. 450 too frequent connects from 198.37.147.135, please try again later. (throttled)


What to do with this response


We’ll continue to retry deferred messages for up to 72 hours for a response like this. Generally this is based on a large influx of messages that you send, or if you've sent at a rate that the recipient server deems worthy of slowing down.



451


The message simply failed, usually due to a server error on the recipient's end.


Example


451 mta1012.mail.gq1.yahoo.com Resources temporarily unavailable. Please try again later [#4.16.1].451 Temporary local problem - please try later


What to do with this response


This is unlikely to be anything you or Questline can prevent. We’ll keep retrying for 72 hours.



452


The message has been deferred due to insufficient system storage.


Examples


452 Too many recipients received this hour (throttled)452. 4.3.1 Insufficient system storage (throttled)452 4.2.2 Over Quota


What to do with this response


There's not much that can be done in these cases. We’ll continue to retry deferred messages for up to 72 hours for a response like this. If space frees up in the subscriber's mailbox, the server will likely accept the mail later. This may indicate the mailbox has been abandoned by your customer.



Hard Failures (Hard and Block Bounces)


A Hard, or Immediate, failure is anything that gets a 500-style message as the result of trying to hand off a message. This typically indicates that some sort of permanent error occurred, and can range from systemic errors on the recipient's server to policy-related blocks pertaining to content or sending patterns. The examples below provide insights on some of the many reasons a 500-level bounce can be returned. 


550


The user's mailbox was unavailable. Usually because it could not be found, or because of incoming policy reasons.


Examples


550 5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces.550 Requested action not taken: mailbox unavailable550 5.1.1 <address@aol.com>: Recipient address rejected: aol.com


What to do with this response


The most common reason this message is returned is because the mailbox no longer exists or the address was mistyped. The Engage system automatically suppresses messages with this response from receiving future messages to protect your deliverability. Sometimes, this message can be returned for a legitimate email when the mail administrator has configured strict anti-spam policies, such as not allowing multiple messages through at the same time from one sender. If you see a bounce message with this code that indicates anti-spam measures, it's best to ask the recipient's mail administrator to add an exception to that policy for your email.



551


The intended mailbox does not exist on this recipient server. This response will sometimes include a forward address to try if the server knows where the intended mailbox is.


Examples


551 not our customer551 user does not exist


What to do with this response


The recipient server does not recognize the recipient address as being one of it’s own. Keep any eye on the human readable portion of the response, as it may include a forwarding address that will allow you to update your customer record. In general, Engage will suppress these addresses from receiving future mail.



552


The intended mailbox has exceeded its storage limits.


Examples


552 5.2.2 This message is larger than the current system limit or the recipient's mailbox is full. Create a shorter message body or remove attachments and try sending it again.


What to do with this response


There's a pretty good chance this is a defunct or abandoned email address. We suppress most subscribers when receiving a bounce with this error code, so refer to the human readable portion for more guidance.



553


The message was refused because the mailbox name is either malformed or does not exist.


Examples


553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)553 Invalid/inactive user.


What to do with this response


It's highly likely this is a bogus or mistyped address. We suppress most subscribers when receiving a bounce with this error code, so refer to the human readable portion for more guidance.



554


The message failed. This response is essentially a default or generic failure. Refer to the human-readable portion of the message for further instruction.


Examples


554 5.7.1 - ERROR: Mail refused554 5.7.1 [P4] Message blocked due to spam content in the message.


What to do with this response


Use the human readable portion of the message for further guidance. If it's unclear, consider the address “bad” unless the recipient contacts you. We suppress most subscribers when receiving a bounce with this error code, so refer to the human readable portion for more guidance.