A Soft Bounce is a messages that makes it all the way to your recipients email account, but is rejected by their WebMail server. This could be due to a number of reasons, the recipients inbox could be full, their WebMail sever may be down, or the messages is too large.
A Hard Bounce is a bounced message that does not make it to the recipients email server and is permanently undeliverable. Causes for this include, domain name does not exist, typos in the email address, or recipients may have blocked your mail server.
You can take steps to reduce bounces from your campaign messages by doing a few things:
1. Test your emails.
2. Keep your contact lists clean
3. Use opt-in for your contact lists to avoid contacts flagging your message as spam