What’s the difference between a soft bounce and a hard bounce?

The difference is permanence. Hard bounces won't resolve. Soft bounces might.

Hard bounces indicate permanent delivery failure:

The email address doesn't exist (user unknown)
The domain doesn't exist or has no mail server
The recipient has permanently blocked you
The address was valid but has been deactivated

Hard bounced addresses should be immediately removed from your list. Continuing to send wastes resources and damages reputation.

Soft bounces indicate temporary delivery failure:

The mailbox is full and can't accept more messages
The receiving server is temporarily unavailable
The message is too large for the recipient's limits
Rate limiting or temporary policy blocks

Soft bounced addresses might accept email later. Most systems retry soft bounces several times before giving up.

Classification isn't always clear. Some errors are ambiguous. A "mailbox full" might be temporary (vacation) or effectively permanent (abandoned account). Some ESPs convert repeated soft bounces to hard bounces after multiple failures.

Track your bounce rates. Industry average is under 2%. Rates above that suggest list quality problems. Rates dramatically above that indicate serious issues requiring immediate attention.