Recently a client asked me what email bounces were on their email campaign reports and how it impacted their email campaign delivery. Understanding that they are not the only one to ask questions and not fully understand what email bounces were I decided to write an article on email marketing bounces and explain the different type of bounces that a company may see upon sending an email newsletter.
There are many different type of email bounces but they will not all show up on a company’s email marketing reports upon sending an email campaign. The reason they may not all show up is how a company sends their email newsletters.
If the company has an internal infrastructure where the email marketing software was developed internally and the backend is completely self managed then they are more likely to see more detailed reports and will run into most if not all bounce types below. Such a setup most likly uses an MTA (mail transfer agent) to handle email delivery and the email bounce reports are generated by the mail transfer agent.
For those not familiar with what an email transfer agent (MTA) is, it is a software that sends emails from one server to another. An MTA takes care of both the sending of the original email message and the receiving portions of the Simple Mail Transfer Protocol. The most popular MTA software is Port 25.
Another way to send email newsletters is through an email service provider such as MailChimp, IContact, Campaign Monitor, ExactTarget or Blue Sky Factory then the email service provider manages the bounce reports and the company sending the email newsletter are not likely to see the detailed bounce types below but are likely to have the email bounces broken down into the following:
Hard Bounces (A hard bounce is an email message that’s returned to the original sender and is permanently undeliverable. While there are many reasons that may cause this, the most common reasons are email addresses that do not exist, the email recipient mail server has blocked emails that are coming from your server due to suspected SPAM activities.
Soft Bounces (A soft bounce might occur when an email message has been sent and the recipient’s email box is full, the server is down or swamped with messages, or the message is too large).
Here’s a list of the different types of email bounces that a company may see in their email marketing reports and a breakdown of the type of bounce that it is.
Auto Reply (AR)
These are what are known as soft bounces which are caused by an automatic response that is generated by the recipient email software, for example “On Vacation” emails messages. In many cases, the email newsletter that was sent gets delivered to the inbox and the bounce would usually be removed from the email marketing reports once the recipient reads the email (If images are enabled or they click one of the links within the newsletter)
General Bounce (GB)
The email server was not able to deliver the email newsletter, but the exact reason for the email bounce could not be determined. In most cases these types of bounces are classified as soft bounces.
An example of the message that may be received is: ”Subject: Undeliverable mail”
Hard Bounce (HB)
This is a type of email bounce where the original newsletter that was sent is permanently undeliverable, but the recipient’s server has not provided a detailed reason for the bounce.
Hard bounces should be removed from an email marketing list as soon as possible to maintain as well as improve a company’s reputation with ISPs. Removing these types of emails from your email marketing list will avoid your company being labeled as a SPAMMER.
Soft Bounce – General (SB)
This type of bounce response is received when an email server is temporarily unable to deliver an email message to the recipient email address. As explained above, this could be for a variety of reasons such as “Connection timed out, the recipient’s email box is full at the time that the email was received or it is also possible that the email server is down.
It is considered industry best practice to remove an email address from an email marketing list if a soft bounce occurs on that email address more than three times.
Soft Bounce - DNS Failure (SBDF)
The email server for the time being is unable to deliver your newsletter to the recipient email address due to a DNS problem.
An example of the message that may be received is: ”Host is unreachable”
Soft Bounce – Mailbox Full (SBMF)
The email server for the time being is unable to deliver your newsletter to the recipient email address because the mail box of the recipient is full.
An example of the message that may be received is: ”Mailbox over quota”
Soft Bounce – Message Size Too Large (SBMS)
The email server was unable to deliver your message to the recipient because the message size is too large. Usually this is due to additional attachments that may accompany the original email message.
An example of the message that may be received is: ”Exceeded maximum inbound message size”
Transient Bounce (TB)
The email server for the time being is unable to deliver the email newsletter but it will continue trying to send it.
An example of the message that may be received is: ”Warning: message still undelivered after 4 hours. We will keep trying until message is 2 days old”
Mail Block – Relay Denied (MBRD)
This indicates that the recipient’s email server is blocking email from the email server that you may be using. This could be for a variety of reasons but most likely if neither the sending mail address nor the recipient’s address are locally hosted by the server,then the ISP’s servers may refuse to relay the message on. User not local or invalid address –Relay denied. This type of error is also called SMTP Error 5.5.1 or SMTP Error 551
An example of the message that may be received is: ”551 relaying denied”
An instance of when this would occur is as follows:
Your company has a domain that is hosted by somehostingprovider.com but your ISP is roadrunner.com and you try to send emails from your domain to email@example.comNeither your domain nor somecompany.com are hosted by roadrunner.com, as a result your email is not accepted by roadrunner.com mail servers and your mail server is returned an SMTP Error 551. To correct this type of issue your company has to contact roadrunner.com and ask them to include your domain name as an allowed sender.
Non Bounce (NB)
This message was not a bounce. This could be the recipient replying or perhaps it was a bounce format that was not recognize by the email marketing software.
Mail Block – General (MB)
Indicates that the recipient’s email server is blocking email from email that is coming from the IP address that is assigned to the email server which your company is using.
There are many reasons why this may occur such as:
- SMTP Error 550 will be returned by many servers. If the recipient email address does not exist on the remote side. The reply messages that will most often be received are “550 Invalid recipient”, “550 User account is unavailable”, “<ip-address-ofremoteserver> does not like recipient – 550 Address rejected” or “550 No such user here” or “550 Not our Customer” or “550 Account not available” or “Remote host said : 550 – firstname.lastname@example.org, this SOMECOMPANY.COM Mailbox Does Not Exist – Giving up”).
- SMTP Error 550 is also returned when sending of email is attempted using a server which requires SMTP authentication and such authentication has not been provided.
- SMTP Error 550 can also be returned by the recipient’s anti-spam firewall if the anti-spam firewall does not see a whitelisting for the sender or their server’s domains/IP addresses. A classic example of such SMTP Error 550 return message by an anti-spam firewall may be :
22.214.171.124 does not like recipient.
Remote host said: 550-Verification failed for
550-Previous (cached) callout verification failure
550 Sender verify failed
Giving up on 126.96.36.199.
Example: ”550 Message REFUSED by peer”
While there are many other reasons why a SMTP Error 550 may occur, the examples above are a few such samples.
Mail Block – Known Spammer (MBKS)
Indicates that the recipient’s email server is blocking all of the email messages that are coming from your servers due to the fact that it believes you are a spammer. This is clearly a reputation issue that needs to be investigated, addressed and corrected.
An example of the message that may be received is: ”REJECT Known SPAM source”
Mail Block – Spam Detected (MBSD)
This indicates that the recipient’s email server is blocking all of the email messages that are coming from your servers due to the fact that it believes the message has content which looks like spam. This falls under SMTP Error 550 which is also known as SMTP Error 5.5.0
An example of the message that may be received is: ”550 Possible spam detected”
If your email is not promoting SPAM then you may want to run your email through SPAM filter software to check which content is raising flags so that you can correct these issues from occurring in the future.
Subscribe Request (SR)
A message has been sent back to the bounce address, looking to be added to your list. In view of the fact that actual email subscribers would not usually know this email address, these types of email messages are just considered soft bounces.
Unsubscribe Request (UR)
These email messages to unsubscribe will be sent to the bounce address and are considered bounces. Email subscribers which receive the email newsletter may click the unsubscribe link or use the ‘reply-to’ address of they wish to be taken off your email list so as to not receive any more email marketing messages.
When you look at your email marketing reports however you will see these as unsubscribe requested and not an email bounce.