Email Bounces Explained with Keap – Keap Max Classic

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. When an email bounces, the recipient's server sends a message back to Keap that describes the reason for the bounce.

You can view the bounce type in the following reports:

For Pro / Max users: go to Reports → Email Engagement Tracker.
For Classic users: go to Marketing → Reports → Email Status Search.

General bounce

A general bounce occurs when a receiving mail server is unable to deliver a message but does not provide a specific diagnostic code explaining why. In most cases, this reflects a soft bounce condition.

Typically, the receiving server initially accepts the message. However, shortly after our servers close the SMTP connection, the receiving system generates a bounce notification. Because the connection is already closed, the bounce is sent out of band and in many cases, it never makes its way back to us. Without that return path, we cannot capture the underlying SMTP error, so the event is recorded as a general bounce.

These Out-of-Band (OOB) bounces are common across the industry and generally indicate temporary issues on the receiving domain’s side (e.g., throttling, reputation filtering, or transient mailbox/server problems).

Hard bounce

A hard bounce is recorded when an email message is considered permanently undeliverable. The email system will not try to deliver the email again. Hard bounce email addresses are automatically disabled. The system will not allow you to send any automated email to these accounts. This can be due to the email address not existing, your mail is being rejected due issues with the authentication of you mail, or the mail is being blocked due to content/reputation related issues. You will also see a Hard Bounce if the email address is on the Restricted sending list. Your sender reputation can be affected by too many invalid hard bounces.

Unlike spam complaint rates, there is currently no agreed upon industry standard rate when it comes to invalid hard bounces. At Keap, we consider anything over 5% per email provider as excessive and at risk for being contacted by our Email Compliance Team.

Soft bounces

A soft bounce occurs when a message cannot be delivered due to a temporary issue with the recipient’s mailbox or receiving server. Keap does not surface soft bounces directly to users. Instead, messages that soft bounce are placed into a 12-hour retry queue. During this window, the system makes multiple delivery attempts.

If delivery is still unsuccessful after 12 hours, the message is removed from the retry queue and marked as a Hard Bounce (Timeout). In these cases, Keap records both the timeout status and the original soft-bounce reason for transparency. Many receiving systems will attempt redelivery internally for hours or days, but from Keap’s perspective, the message becomes undeliverable once the retry queue expires.

Common Types of Soft Bounces

Mailbox Full
The recipient’s mailbox has exceeded its storage quota. This is typically due to poor mailbox maintenance, but it may also indicate an inactive or abandoned account.
Message Too Large
The email exceeds the maximum acceptable size of the receiving system, usually due to large attachments or embedded content.
DNS Failure
The receiving server cannot be located or cannot resolve DNS records needed for delivery. This usually reflects DNS misconfiguration on the recipient’s domain rather than an issue with the sender. (SPF/DKIM/DMARC misalignment can also contribute in some cases.)
General
A temporary delivery failure occurred, but the receiving system did not provide a specific SMTP error code.

Mail blocks

A mail block is recorded when the receiving mail server rejects a message before attempting inbox delivery. This is an immediate SMTP-level rejection, not a temporary failure. When a mail block occurs, Keap does not retry delivery. However, the recipient’s marketability status is not affected, and you may manually resend the message once the underlying issue is resolved.

Types of Mail Blocks

General
The receiving server is blocking messages originating from Keap’s sending infrastructure. No specific reason is provided.
Known Spammer
The receiving server is rejecting your message due to sender reputation issues tied to your domain, IP, or historical sending patterns. This indicates the recipient’s system believes your mail resembles spam.
Relay Denied
The receiving server will not accept the message for relay from Keap. Ensuring your SPF record includes infusionmail.com may help reduce these rejections by clarifying authorized sending sources.
Spam Detected
Content-based filters on the recipient’s server are identifying your email as spam. Use Keap’s Spam Score tool in the email builder to evaluate and reduce potentially suspicious content. A score under 5 ideally 0 is recommended.
Attachment Detected
The receiving server rejected the message due to the attachment. Possible causes include:
- The file type is restricted (e.g., .exe).
- The system does not allow attachments at all.
- The attachment was flagged as a potential malware source.
- The file size exceeds limits (ensure attachments stay under 10 MB).

These blocks indicate policy-based rejection by the receiving system and typically require adjustments to your content, domain setup, or sending practices.

Unsubscribe request

These are recorded when an auto-reply request is sent to your bounce capture email account(mailer@infusionsoft.com or bounce@infusionsoft.com) asking to be removed from your email list. 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.

Undetermined

An undetermined status is assigned when Keap is not able to identify the cause of the bounce based on the feedback received from the receiving server.

Invalid Address Bounces

An Invalid Hard Bounce occurs when you attempt to send email to an address that does not exist or is otherwise undeliverable at the receiving mailbox provider (MBP). The MBP returns a permanent failure (5xx) response indicating the address is invalid. Future attempts to send to that address will always fail

Potential causes of invalid addresses

Sending to a Cold Email List
- A cold email list is one where there has been a significant amount of time since an email has been sent.
- Many customers change their email address every year and may change their address without you knowing.
- Sending regular emails helps catch customers before they change so they can resubscribe with their new address.
Purchased or Scraped Lists
- Purchase or scraped lists typically have a significant amount of invalid email addresses.
- It is better to generate all email lists organically through lead collection.
- Warning: Any account known to have imported a purchased or scraped list is in danger of having their account suspended.
Invalid Lead Generation Information
- Web forms that include an email address can be the source of potential invalid email addresses. Fake email addresses are frequently entered into web forms to get to the promised access to information.
- This typically happens when the potential customer isn't interested in getting spam from the company but really wants the content or promotion.
List Bombing
- List bombing happens when a web bot randomly fills out web forms with obscure email addresses.

Resolution

If you have received an email from the Keap's Compliance Team about excessive invalid hard bounces being generated by your application then please read this section carefully and enact the measures needed to reduce your hard bounce rate.

List hygiene
Audit your landing pages and check for unsecured web forms
Audit your lead collection process for incoming invalid email addresses and take the steps needed to promote trust and loyalty to your prospective leads
Review the best practices guide and make improvements where necessary

Looking for extra help?

If you’d like professional guidance with your email practices or recommended tools to improve your email practices and deliverability, check out these trusted partners:

Email Deliverability specialist training, consulting and software
- EmailSmart
List Cleaning
Form Security
- Spamkill
- ListDefender