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Emails "bounce" when they fail to deliver or are rejected by the recipient’s inbox provider. Your campaign's bounce report includes SMTP (Server Mail Transfer Protocol) replies explaining why an email bounced. These replies may indicate issues such as an invalid email address, technical errors, content filtering, reputation concerns, email frequency, unavailable mailboxes, or unclassified reasons.
Marsello categorizes bounces as either hard or soft:
Hard bounces (permanent issues) prevent future email attempts.
Soft bounces (temporary issues) allow up to three more delivery attempts before being marked as a hard bounce.
This guide breaks down common SMTP replies and helps you better understand your bounce report.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 causes of hard bounce emails
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.
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.
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 that indicate a hard bounce
Hard bounces occur when an email cannot be delivered permanently. These email addresses should usually be removed from your mailing list to protect your sender reputation.
550 — Mailbox Unavailable
Bounce reason:
Requested action not taken: mailbox unavailable
What it means:
The recipient’s mailbox could not be found or is unavailable due to mailbox or server policies.
Recommended action:
Remove the email address from your list. The address may be fake, invalid, or mistyped.
551 — User does not exist
Bounce reason:
User does not exist
What it means:
The recipient mailbox does not exist on the destination server.
Recommended action:
Remove the email address from your list.
552 — Mailbox full or message too large
Bounce reason:
Message exceeds system limits or recipient mailbox is full
What it means:
The recipient’s mailbox has exceeded its storage limit, or the email content is too large.
Recommended action:
Try shortening the email body or removing attachments
Marsello does not automatically resend emails with this error because it often indicates an abandoned email account
553 — Invalid or Malformed email address
Bounce reason:
Mailbox name invalid
What it means:
The recipient email address is malformed or does not exist.
Recommended action:
Remove the email address from your list.
554 — Mail refused
Bounce reason:
Mail refused
What it means:
This is a general rejection error that can occur for several reasons. Some servers include additional human-readable details explaining the issue.
Recommended action:
If no additional explanation is provided, remove the email address from your list.
Common causes of soft bounces
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.
Technical error
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.
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.
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.
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 soft bounce
Soft bounces occur when an email cannot be delivered temporarily. Marsello will continue retrying delivery for up to 72 hours unless the issue is resolved sooner.
403 — Unauthorized Sender Address
Bounce reason:
You are not authorized to send from that email address
What it means:
The sender (“from”) email address does not match a verified Sender Identity.
Recommended action:
Verify your sender email address before attempting to send again.
421 — Message Temporarily Deferred
Bounce reason:
Message temporarily deferred
What it means:
The recipient server has temporarily delayed delivery, often due to:
Too many messages being sent in a short timeframe
Too many simultaneous connections
Recommended action:
Marsello will retry delivery for up to 72 hours
Reduce sending volume temporarily to affected domains to avoid further delays
450 — Mailbox Unavailable Temporarily
Bounce reason:
Too frequent connects from IP address, please try again later
What it means:
The recipient mailbox was temporarily unavailable, locked, or unreachable at the time of sending.
Recommended action:
Marsello will continue retrying delivery for up to 72 hours
Consider reducing send frequency to the affected domain
451 — Temporary Server Error
Bounce reason:
Temporary local problem — please try later
What it means:
The email failed due to a temporary issue on the recipient server.
Recommended action:
Marsello will automatically retry delivery for up to 72 hours.
452 — Recipient Limit or Storage Issue
Bounce reason:
Too many recipients received this hour (throttled)
What it means:
The message was deferred due to:
Recipient server throttling
Insufficient system storage
Recommended action:
Marsello will continue retrying delivery for up to 72 hours.
Black Code or “Other” — Unclassified Error
Bounce reason:
Unclassified error code
What it means:
The delivery failure could not be classified or the recipient server did not provide enough detail.
Recommended action:
Marsello will continue retrying delivery for up to 72 hours.