My valid email addresses hard bounced. What happened?

If you send a campaign and it hard bounces back from an email address that you know is a valid address, there could be several reasons why this happened. The first thing to do is read its "bounce header", also known as an "SMTP reply". This information is available in the exported excel/csv list from the bounced report for that email campaign. The server that bounced it back might have inserted some explanation on why it bounced. Examples of bounce header: 554 delivery error: dd This user doesn't have a yahoo.com account (ddddeeeffff@yahoo.com) [0] - mta236.mail.re3.yahoo.com (in reply to end of DATA command) 550 5.1.1 unknown or illegal alias: ddddeeeffff@verizon.net (in reply to RCPT TO command) If your email was denied, the most likely reason is it was blocked by a spam filter in your recipient's email client. If this happens, you will see an SMTP reply that says something like "denied", "unacceptable content" or "blocked". Spam filters can be on the recipient's computer, in their firewall, or at their ISP/data center, or sometimes even all of the above.