A soft bounce is an email message that gets as far as the recipient's mail server (it recognizes the address) but is bounced back undelivered before it gets to the intended recipient. A soft bounce might occur because the recipient's mailbox is full, the server is down or swamped with messages, ...
If an email bounces for a temporary reason (ie. the mailbox is full), we'll won't remove the email address. However, permanent reasons will cause the email newsletter service to remove your subscriber indefinitely.