What is a hard bounce?
A hard bounce is a permanent delivery failure. The message can never be delivered to that address. Retrying will not help, and continuing to send hurts your sender reputation.
- The address does not exist or was mistyped.
- The domain does not exist or cannot receive mail.
- The recipient's provider has permanently blocked your message.
What is a soft bounce?
A soft bounce is a temporary failure. The address is real, but the message could not be delivered this time.
- The recipient's mailbox is full.
- The recipient's provider is temporarily unavailable.
- The message is too large.
- The provider is deferring mail from senders it does not yet trust.
What to do about each
- Hard bounce: remove the address from your list straight away.
- Soft bounce: retry a few times, then suppress the address if it keeps failing over several sends.
- Rising bounces of either kind: pause, find the source, clean the list, then resume gradually.
How to prevent both
Most hard bounces are avoidable. Check addresses at sign-up, correct typos, confirm new subscribers and clean old lists before mailing them. Soft bounces are harder to prevent, so watch the pattern and suppress addresses that keep failing.
Bounce reasons in plain English
- "User unknown" or "mailbox not found": a hard bounce. The address does not exist.
- "Domain not found": a hard bounce. The domain does not exist or cannot receive mail.
- "Mailbox full": a soft bounce. The person may free space later.
- "Message too large": a soft bounce. Send a smaller message.
- "Blocked as spam": not about the address. It is about your message or your sender reputation.
Blocks are a different problem
When a provider refuses your message because it looks like spam, the address itself may be perfectly good. Treat blocks separately: review your content and your sending habits instead of deleting the contact.
Setting up suppression rules
- Remove hard bounces automatically and keep a suppression list so they cannot be imported again.
- Suppress an address after several consecutive soft bounces.
- Give suppressed addresses a re-check after a few months before you decide.
Reading your bounce report
Look at bounce rate per campaign, split hard from soft, and watch the trend. One bad campaign points to a specific list or source. A slow climb across every campaign points to an ageing list.
What healthy numbers look like
As a rule of thumb, aim for a total bounce rate well under 2 percent and treat any sudden jump as a reason to investigate. Hard bounces should be rare on a list you maintain, because each one means you sent to an address that could never receive your message.
If you see a high rate of soft bounces from one company or provider, the cause may be on their side and may clear up. If the same addresses keep failing send after send, stop trying and move them out of your active list.