Bouncy Ball: Email Bounces and Their Types

If you've been into email marketing for at least one day, there's no way you haven't dealt with email bounces. In fact, most of internet users who use email encounter bounced emails on daily basis. A bounced email is one that never arrives in the recipient's inbox; it is bounced back to the sender with an error message indicating that the email was never successfully sent.

Most emails bounce because of a permanent issue with the receiving email account, a temporary issue with the receiving email account, or because the email is blocked by the receiving server.

Bounce Type	Description
NON BOUNCE	Non Bounce
HARD BOUNCE	Hard Bounce
SOFT BOUNCE	General Soft Bounce
SOFT BOUNCE	DNS Failure
SOFT BOUNCE	Mailbox Full
SOFT BOUNCE	Message Size Too Large
GENERAL BOUNCE	General Bounce
MAIL BLOCK	General
MAIL BLOCK	Known Spammer
MAIL BLOCK	Spam Detected
MAIL BLOCK	Attachment Detected
MAIL BLOCK	Relay Denied
AUTO REPLY	Auto Reply
TRANSIENT BOUNCE	Transient Bounce
UNSUBSCRIBE REQUEST	Unsubscribe Request
CHALLENGE RESPONSE	Challenge Response

If you've been using our bulk email service, you know that if your email bounces, the reason is available in the recipients tab (the "campaign status" column).

General Bounce A general bounce means that the server could not deliver an email message, and also could not detect a specific reason for the message’s return. In most cases, this is related to a soft bounce. Example: "Connection timed out" Hard Bounce A hard bounce means that an email is considered permanently undeliverable. In most cases, a hard bounce indicates that the recipient email address does not exist. Often, these are old and expired addresses; however, it may also indicate that your emails are being rejected due to the SPF configuration on your DNS account. You may need to configure the SPF records to allow servers to send email from you. Example: "550 User Unknown" If your email bounces with this error message, review your contact’s email address, and see if there are any evident typos. If not, try to reach the recipients by other means to approve the address. Soft Bounces A soft bounce means that there is a temporary issue with the recipient's email account, which is delaying message delivery. The email system will try to deliver the message several times over a period of hours or days, and will only consider it undeliverable after the retry process times out. There are several types of soft bounces.

• Mailbox Full: The email server is temporarily unable to deliver your message to the recipient email address because the recipient's email inbox is full. This might be related to incorrect maintenance, but it could mean that the recipient no longer actively uses the email account, even though it still exists.

Example: "Mailbox over quota" As with a nonexistent email address, you may want to contact the recipient by phone to check if the address is valid.

• Message too large: If your email bounces and results in the following error, content in the email or attachments exceeds the size limits of the receiving server.

Example: "Exceeded maximum inbound message size"

• DNS Failure: The email server is temporarily unable to deliver your message to the recipient email address because of a DNS problem. This is most likely due to an issue with the settings for your domain, or related to the SPF records.

Example: "Host is unreachable" You can contact your domain administrator for help.

• Auto-Reply: This bounce means that the message has been delivered, but the recipient has activated an automatic response. The bounce status will be removed as soon as the recipient opens the email.

Examples: "Out of Office"; "Vacation Message" Observe how often this email address ends up in this category. If months go by, and the person is still “out of office,” you may want to remove that contact from your mailing list.

• Subscribe Request: This means that someone is requesting to be added to your opt-in mailing list. These requests are recorded when an auto-reply is sent to your bounce capture email account.

Mail Blocks A mail block is recorded when the recipient's email server blocks an email message completely. The server rejects the message before even trying to deliver it to the inbox.

• General: The recipient's email server is blocking email from your email server.

Example: "550 Message REFUSED by peer"

• Known Spammer: If sent email bounces and results in this message, it means you have a real problem to work on. The recipient's email server is blocking messages from your email account because the message appears to have content that looks like spam.

Example: "REJECT Known SPAM source"

• Relay Denied: The recipient's email server is blocking messages sent through your server.

Example: "551 relaying denied"

• Spam Detected: The recipient's email server is blocking messages from your email account because the message appears to have content that looks like spam.

Example: "550 possible spam detected"

• Attachment Detected: The recipient's email server is blocking the message because of the attachment. The attachment might be identified as a possible virus. The system may not allow attachments at all, or may block specific types of files. The size of the attachment may also be causing trouble. Make sure your attachment size is less than 10 MB.

Example: "552 Disapproved attachment" Unsubscribe Request The recipient is requesting to be removed from future email from you. A real person will reply to the email or click on the unsubscribe link. These unsubscribe requests are the same as an ISP spam complaint. Transient Bounce This bounce is recorded when the email server cannot deliver your message, but it is still trying, and will continue to try for a limited period of time. Examples: "Warning: message still undelivered after 4 hours. Will keep trying until message is 2 days old" Undetermined This status is given when the cause of the bounce cannot be identified based on the feedback received from the receiving server. Challenge-Response Challenge-Response email systems were created as a reaction to the increasing circulation of spam. It is an automatic response from the recipient, requesting that the sender confirm that a real person is sending the message. Generally, confirmation is completed manually by entering a code or clicking on a hyperlink. Hopefully, this short explanation of different types of email bounces will be of use for you the next time you push the "send" button on your email campaign.