An email is considered hard bounce if it cannot be delivered permanently. For example if the email address is invalid, the user doesn’t exist, there’s a mispelling or a deleted mailbox, or the domain name is not registered.
We also invite you to read the different case scenarios and categories we have for “soft bounce” in our Email Marketing Software to have a better vision of all possibilities of undelivered email, or bounce.
There are several cases of bad-mail. We analyze several kinds of answers from SMTP servers which we will consider as bad-mail, and we will then classify that email in our bad-mail list.
From those answers, we mainly have:
/domain no longer in use
In this category we will also analyze the answer we will receive from SMTP severs, and find answers with keywords such as :
/email (No such|bad|invalid|unknown|illegal|unavailable|not known|not here|not found|)
/no longer (valid|available)
/Recipient address rejected
It is the same for this category, were the mail box exists but is no longer active, and we cannot deliver messages.
With this categories, we cover all rejected emails in the hard bounce process.