There 3 categories as following for bounces.
Hard Bounces - A hard bounce means the email does not exist anymore on the target server, or it has been blocked/disabled or the target server does not exist, etc.
Soft Bounce - Unlike hard bounces, soft bounces can happen for reasons like server/account temporarily unavailable, not enough disk space to store the email on the server, or
the receiving server is too busy etc.
Internal Bounce - Your content is spammy, your IP/Server/domain listed in the blacklist, the range of IPs are listed in the blacklist, the link present in your email content is
listed, subject lines are considered to be spam, etc. (Related to your email content or your IP/domain's reputation.)