Emails "bounce" when they fail to be delivered or are rejected by the recipient's inbox provider. In your campaign's bounced report you will notice different SMTP (server mail transfer protocol) replies explaining why the email has bounced. SMTP replies in your reporting may include invalid email address, technical reason, content reason, reputation, frequency/volume of emails going out, mailbox unavailable and unclassified.
Once an SMTP reply is noted, Marsello will review this information and further classify the email address as a hard bounce or a soft bounce.
Email addresses classified as hard bounce will not be sent further emails as there is a permanent delivery issue (invalid email address or the mailbox provider is permanently blocking delivery).
Email addresses classified as a soft bounce (temporary delivery block) will see three more attempts to deliver the message before being marked as a hard bounce.
This guide will walk you through some of the most common SMTP replies and offer insights to help you better understand your bounced reporting.
Hard bounce SMTP replies
Hard bounces indicate permanent delivery failure to a recipient's email address. This often occurs due to an error in the spelling of the recipient's email address or when the recipient mailbox doesn't exist.
Common errors that cause emails to hard bounce
1. Invalid Address
An invalid address message indicates the recipient's email address is syntactically incorrect or the mailbox does not exist. It is possible for emails that were previously valid to become invalid as mailbox providers purge mailboxes that haven't been used in a long time.
💡Tip: It is recommended to keep hard bounces to fewer than 5% of your attempted sent messages to prevent a negative impact on your sending reputation.
2. Content
A content message indicates that the mailbox provider believes the content of your messages is potentially spam or untrustworthy. Some common reasons for mailbox providers to distrust an email message include:
Suspicious links or URLs within the content.
Many mailbox providers will reject messages that contain link shorteners such as Bitly.
Message content is too large.
Some mailbox providers have a maximum message size. This limit varies from provider to provider. For example, Gmail has a max message size of 25Mb. Hotmail will reject messages larger than 10Mb. Smaller mailbox providers or legacy mailbox providers can have size limits as low as 2Mb.
Attachment too large, no attachments allowed, or invalid attachment type.
Mailbox providers can reject messages that contain any attachments. They can also reject messages that contain attachments that are too large or messages that contain specific types of attachments.
Text to Image ratio
When an email message contains just a single image and no accompanying text, it may look like the sender is trying to disguise the content of the message. Avoid sending messages that contain mostly images and very little text.
Spam or malicious content.
This type of content rejection occurs when the message you sent closely resembles known spam or malicious email. Mailbox providers and spam filtering companies are constantly reviewing which messages generate negative feedback from their users. This is a moving target. What recipients were marking as spam last year might be totally different than what they are marking as spam today.
3. Reputation
A reputation message signifies that the mailbox provider detects reputation issues with your sending domain or IP. Providers gauge your sender reputation through intricate algorithms considering numerous signals. Pinpointing the precise reason for a poor reputation is often challenging, yet it's predominantly influenced by
recipient engagement (positive vs. negative)
Positive recipient engagement includes things like recipients replying to a message, opening a message, moving an email out of the spam folder, marking a message as important, or adding an email address to a contacts list.
Negative recipient engagement includes things like spam complaints, marking a message as phishing, deleting a message without reading it, or not opening an email from a specific sender for an extended time.
sending behavior
Sending email to recipients who have not engaged with your email for an extended time.
Sending email that your recipients are not interested in receiving.
Failing to remove recipients who mark your messages as spam from your list.
Not properly warming your IPs and domains.
Dramatically increasing your sending volume.
Sending to recipients who haven’t opted in to receive your email with explicit and informed consent (sending to purchased, rented, or scraped addresses).
SMTP error codes in reporting indicating a hard bounce
SMTP codes on reporting | Bounce Reason | Explanation |
550 | Requested action not taken: mailbox unavailable | The user’s mailbox was unavailable. Usually because it could not be found, or because of incoming policy reasons. Remove these address from your list - it is likely a fake, or it was mistyped. |
551 | User does not exist. | The intended mailbox does not exist on this recipient server. Remove these addresses from your list. |
552 | 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 | The recipients mailbox has exceeded its storage limits. We don't resend messages with this error code because this is usually a sign this is an abandoned email |
553 | The recipient's mailbox has exceeded its storage limits. We don't resend messages with this error code because this is usually a sign this is an abandoned email | The message was refused because the mailbox name is either malformed or does not exist. Remove these addresses from your list. |
554 | ERROR: Mail refused | This is a default response that can be caused by a lot of issues. There is often a human-readable portion of this error that gives more detailed information, but if not, remove these addresses from your list. |
Soft bounce SMTP replies
A soft bounce in email reporting refers to a temporary failure to deliver an email to a recipient's inbox. Marsello will send messages to an email address that has a soft bounce up to three times before classifying the bounce as a hard bounce.
💡Tip: It is recommended to keep a close eye on your bounce rate to be sure your emails are reaching your customers. This can also help you to abide by spam laws and avoid bounce suspensions.
Common errors that cause emails to soft bounce
1. Technical
A technical message indicates the message failed to deliver due to a technical error. This error can be on the sender's side or the mailbox provider. Often, it's hard to determine the source of the technical rejection. Some common causes of a technical rejection include:
The sender has missing or misconfigured DNS records.
The sender is failing authentication.
Temporary DNS or network failures.
Terminated connection between sender and receiver.
Service outages at the mailbox provider.
💡 Tip: To help ensure the deliverability of your messages, it is recommended to create a custom domain.
2. Frequency/Volume
The frequency/volume too high message indicates the mailbox provider is unable to process the number of messages you are trying to send. This could be due to excessive quantity or speed. Each provider has different capacities; while larger ones like Gmail can handle high volumes swiftly, smaller ones may reject emails temporarily to safeguard their systems. This issue can also impact your reputation as a sender.
3. Mailbox Unavailable
The "mailbox unavailable" classification message indicates the recipient's mail server can't deliver the email due to issues with the individual recipient's mailbox. This could be due to:
The mailbox being full, unable to accept more emails.
Deactivation due to prolonged inactivity, leading to the mailbox becoming invalid.
Temporary deactivation by the recipient.
Suspension by the mailbox provider, often due to payment issues or spam concerns.
3 . Unclassified
While most mailbox providers offer clear rejection messages, some messages are returned without a clear reason. When rejection messages like this are returned, they are placed into the Unclassified bucket.
SMTP error codes in reporting indicating a hard bounce
SMTP codes on reporting | Bounce Reason | Explanation |
403 | You are not authorized to send from that email address | This means the "from" address does not match a verified Sender Identity. Mail cannot be sent until this error is resolved. |
421 | Message from (X.X.X.X) temporarily deferred | Messages are temporarily deferred because of recipient server policy - often it's because of too many messages or connections in too short of a timeframe. We continue to retry deferred messages for up to 72 hours. Consider temporarily sending less messages to a domain that is returning this code because this could further delay your messages currently being tried. |
450 | Too frequent connects from IP address, please try again later | The message failed because the recipient's mailbox was unavailable, perhaps because it was locked or was not routable at the time. We continue to retry messages for up to 72 hours. Consider temporarily sending less messages to a domain that is returning this code because this could further delay your messages currently being tried. |
451 | Temporary local problem - please try later | The message simply failed, usually due to a far-end server error. We continue to retry messages for up to 72 hours. |
452 | Too many recipients received this hour (throttled) | The message has been deferred due to insufficient system storage. We continue to retry messages for up to 72 hours. |
Black code or "other" indicated on reporting | Unclassified error code | Unknown why the message failed to deliver. We continue to retry messages for up to 72 hours. |