Email Bounce Messages Explained: Hard vs Soft Bounces

You send an email and get an automated "delivery failed" reply - a bounce. Bounces are normal, but understanding them helps you fix delivery problems and protect your sender reputation. Here is what they mean.

What is an email bounce?

A bounce is an automated message from a mail server telling you a message could not be delivered. It usually includes a reason and a code. Bounces come in two main types: hard and soft.

Hard bounces

A hard bounce is a permanent failure - the message will never be delivered to that address. Common causes:

• The address does not exist (typo, or the mailbox was deleted)
• The domain does not exist or has no mail server
• The recipient's server permanently rejected it

What to do: remove hard-bounced addresses from your lists immediately. Repeatedly emailing dead addresses harms your sender reputation and deliverability.

Soft bounces

A soft bounce is a temporary failure - delivery might succeed on a retry. Common causes:

• The recipient's mailbox is full
• The receiving server is temporarily down or busy
• The message is too large
• Temporary greylisting (see what is greylisting)

What to do: usually nothing - mail servers retry automatically for a period. If soft bounces persist for the same address, treat it like a hard bounce.

Common bounce reasons at a glance

• "User unknown" / "No such user" - hard bounce; address is wrong
• "Mailbox full" - soft bounce; try later
• "Message too large" - soft bounce; reduce attachment size
• "Blocked" / "Spam" - your mail was filtered; a deliverability/authentication issue

Bounces and your reputation

High bounce rates - especially hard bounces - signal to mail providers that you may be sending to poor lists, which hurts deliverability for all your mail. Keep lists clean and act on hard bounces promptly. More on this in how to improve email deliverability.

Prevent bounces at the source

Many "blocked" bounces come down to authentication. Setting up SPF, DKIM and DMARC correctly stops your legitimate mail being rejected as suspicious - see email authentication explained.

We help you land in the inbox

DigiCloudMail sends from clean, well-reputed infrastructure and helps you configure authentication properly, so bounces stay low and your mail gets delivered.

Want reliable delivery and fewer bounces? Get in touch.