A bounced email is an electronic message returned to the sender because it was not delivered to the recipient for some reason. To understand possible reasons of email bouncing, let's go deeper into theory and trace the path the message takes from the sender's computer to the recipient's Inbox.
To reach the target recipient, the email message must successfully pass through numerous phases. Your email newsletter software sends the emails to your SMTP server or to your ISP SMTP server (it depends on the SMTP server you use). The SMTP server gathers the emails and puts them in a queue, then it sends the messages to the SMTP servers of your recipients' Internet Service Providers by establishing a conversation with the SMTP servers, i.e by connecting to the port 25 on the recipient's SMTP server. The recipient's SMTP server tells the sender's server if it is ready to receive the message or not. If it is not, the sender's mail server terminates the connection and will try again later.