An email bounce happens when the email can't be conveyed to the email address you proposed to send it to.
Hard Bounce: Hard bounce is an email message with a perpetual conveyance failure. It could be because of a mistaken or invalid email address, old domain, and so forth.
Soft Bounce: Soft bounce is an email with an impermanent conveyance failure. This could be because of a brief issue with the beneficiaries' server like a full post box, composed incorrect email address, and so forth.