The Hidden Cost of Hard Bounces

A "hard bounce" occurs when you send an email to an address that doesn't exist. It seems harmless—the email just didn't go through, right? Wrong.

The Blacklist Threat

Repeated hard bounces are a primary trigger for spam traps and blacklists. If you hit a spam trap (an old, abandoned email address used by ISPs to catch spammers), your entire domain could be blacklisted. This means NONE of your emails will get through, even to valid users.

The Account Suspension Risk

ESPs like Mailchimp and SendGrid have strict thresholds. If your bounce rate exceeds 5%, they may suspend your account immediately to protect their own IP reputation.

Prevention is Simple

The only way to prevent hard bounces is to verify your list before sending. Kaiju checks the MX records and performs an SMTP handshake to ensure the mailbox exists and is accepting mail.