Hard bounces and soft bounces are messages that ISPs send back to Acoustic Campaign to explain why they cannot deliver your email to the intended recipient. When an email cannot be successfully delivered, it is considered a bounce.

Bounces are one of the key pieces of data that ISPs use to determine IP address and domain reputation. Bounce handling is a critical part of list hygiene and management.

How does the Acoustic Campaign handle bounces?

When an email bounces several times in a row, a process in Acoustic Campaign flags it for suppression.

• The number and type of bounces required to trigger suppression varies.
• This process prevents the continued sending to invalid email addresses.

Review both soft and hard bounces regularly, either manually or by automation. Continuing to send to invalid addresses is considered a dangerous practice and results in negative responses from ISPs (such as throttling, spamfoldering, and blocking).

Soft bounces and temporary failures

A soft bounce occurs when the email server rejects an email due to a temporary condition, such as a full inbox. When a soft bounce occurs, the system tries to send the email again until it is either accepted or times out. The timeout is set in Acoustic Campaign.

For soft bounces, you can continue sending to these subscribers for a short time to see whether the situation changes. However, if delivery issues persist, cease email and contact the subscriber through other methods to confirm the address. If the address cannot be verified, it should be permanently suppressed.

Warning: Every email contains four addresses, Email recipient, "From", "Reply to", and Bounce. It's possible an email might bounce and the bounces are returned to the Acoustic Campaign bounce server, but you are not notified. In this case, it's possible the email server sent the delivery failure to your "reply to" address and ignored the bounce address. This problem happens occasionally and is ultimately determined by the receiving email server.

Inactive accounts

An inactive account indicates that the email address is temporarily unavailable because the recipient's mailbox is inactive or temporarily disabled.

Mailboxes can be disabled for several reasons (either temporary or permanent):

• Infrequent use
• Change of email address
• Administrative reasons

Mailbox full

The recipient's mailbox is full or exceeded storage allocation, which can be caused by:

• Infrequent use
• A temporary change in email use (for example, a vacation or change of jobs)
• An abandoned mailbox
• Change of email address

Temporary domain failure

There is a temporary failure at the receiving domain, which can occur for technical reasons, such as:

• Failure to renew the domain
• Network or routing issues

ISP blocks

ISP blocks occur when the receiving email server temporarily rejects the email for policy reasons, such as:

• URL or content blocks
• Lack of proper authentication
• Poor reputation
• Presence of the sending IP address or domain on a blacklist

ISP blocks are also called "tempfails" or "4xx series errors" and are transient. For example, Yahoo often uses these blocks when a sender is generating too many complaints, sender reputation is poor, or email server resources are overtaxed.

These addresses should not necessarily be removed from the email list. If the block is due to a poor quality email list, review the list and correct any problems before you send again. If necessary, the Acoustic Campaign Deliverability team can provide assistance.

Hard bounces and ISP blocks

A hard bounce occurs when the email server rejects the email due to permanent conditions. This bounce typically results when "domain not found" or "user unknown" errors occur.

Domain not found

"Domain not found" means that the domain is bad or non-existent. Sending to non-existent domains is a direct result of:

• Poor data acquisition methods
• Old data
• Questionable data sources

When you use old contact lists, you increase the chance of sending to non-existent domains, which is dangerous because non-existent domains can be spam traps.

User unknown

"User unknown" means that the address is invalid. Receiving a "user unknown," "user invalid," or "recipient address rejected" error indicates that the address is either no longer active or was never valid. When you use old contact lists, you increase the chance of sending to unknown users. The percentage of unknown users in a mail stream is a metric that ISPs use as an ingredient in their spam filters. Keep your email list as clean and fresh as possible.

Bad address syntax

The bad address syntax error is returned when an email address is not syntactically correct. It might be missing a @ sign or a domain, or it might contain invalid characters.

ISP blocks

If an ISP issues a block that is not temporary, it indicates that they no longer accept mail from the sending IP address. In this case, the email address will be added to the master suppression list with an undeliverable status. Determine what caused the block and correct the problem before you open a ticket with the ISP.

Other

Another type of block can occur when an email address is invalid or the failure is permanent. However, the bounce message does not provide additional information.

BCC addresses - hard or soft bounces and opting out

If you use blind copy (BCC) addresses in your emails, there are additional issues to consider in regard to bounces and opt-out.

Soft bounces

Campaign keeps a record of addresses that soft bounce. However, Campaign cannot determine whether the BCC or the recipient's address bounced. Also, Campaign does not resend the email or retain a copy of the message because bounce retries are disabled.

Hard bounces

If the BCC send message hard bounces when the target inbox is unavailable, all recipient emails are marked as undeliverable. Depending on your suppression settings, the recipients might be added to your organization's master suppression list.

Important: Work with your IT team to configure a fail-safe when your BCC inbox is unavailable, such as a backup inbox.

Opting out

If a recipient opts out of a BCC email, that recipient is opted out of the list and their email address is copied to the master suppression list.