The short definitions
Soft bounce — temporary failure. The address exists; delivery could not happen right now.
Hard bounce — permanent failure. The address does not exist, or the domain does not.
Common causes of a soft bounce
- The recipient's mailbox is full
- Their mail server was temporarily down or overloaded
- The message exceeded a size limit
- The receiving server applied temporary rate limiting
- A content filter deferred the message rather than rejecting it
Most email platforms will retry a soft bounce automatically over a period, and many eventually deliver.
Why the distinction matters
Hard bounces must be removed immediately. Continuing to mail an address that does not exist signals to providers that you are not maintaining your list, and that reputation damage affects every email you send afterwards.
Soft bounces should be watched, not immediately removed. A one-off is normal and means nothing.
The rule that works: if an address soft bounces repeatedly across several consecutive sends, treat it as effectively dead and remove it. A mailbox that has been full for two months is not being read.
What a rising soft bounce rate is telling you
An occasional soft bounce is routine. A sudden increase usually indicates something worth investigating:
A deliverability problem. Some providers defer messages from senders they are unsure about, which appears as soft bounces before it becomes outright blocking.
An authentication issue. Particularly after a platform migration, where SPF or DKIM records still point at the previous sender.
A list quality problem. Old lists accumulate abandoned mailboxes that fill up and stay full.
A content issue. Very large emails, usually caused by uncompressed images.
What to do about them
Most of this should be automatic in a competent platform:
- Hard bounces removed on the first occurrence
- Soft bounces retried, then removed after several consecutive failures
- Bounce rate watched between sends for sudden movement
- Any sharp rise investigated before the next campaign rather than after
The practical view
Soft bounces are not usually a problem in themselves. They are a useful early warning that something upstream is going wrong, and the businesses that watch them catch deliverability problems before their campaigns quietly stop arriving.
Checking yours
Look at your last three sends and compare bounce rates. A trend upward is worth acting on now rather than later.
Frequently asked questions
What causes a soft bounce?
A full mailbox, a server temporarily down, a message over a size limit, rate limiting, or a filter deferring rather than rejecting. Most platforms retry automatically and many eventually deliver.
Should I remove soft-bounced addresses?
Not on the first occurrence. If an address soft bounces across several consecutive sends, treat it as dead and remove it — a mailbox full for two months is not being read.
What does a rising soft bounce rate mean?
Often a deliverability problem building — some providers defer mail from senders they are unsure about before blocking outright. It can also indicate an authentication issue after a migration.
How is a hard bounce different?
It is permanent — the address or domain does not exist. Hard bounces must be removed immediately, because continuing to mail them damages your sending reputation.