What is Hard Bounce/Soft Bounce?
An email that is rejected by an email server for a permanent reason. An email may hard bounce if a recipient email address or domain name doesn’t exist; the recipient email server has completely blocked the delivery. There are, however, occasionally times when a valid email address will hard bounce.
Example
If you attempt to send an email to an address like “invalid@example.com” and receive a hard bounce notification stating “User does not exist,” it means the recipient’s email address is invalid or non-existent, and the email cannot be delivered.
If you attempt to send an email to an address like “user@example.com” and receive a soft bounce notification stating “Mailbox is full,” it means the recipient’s mailbox is currently full, and the email cannot be delivered. However, you may try sending the email again later when the recipient has cleared space in their mailbox.