Click Mail Servers. If not, select it and click delete and continue with step 6, otherwise skip to step 11. E-mail messages that bounce back are undeliverable and will give you an error message. The most common cause of e-mail being returned to sender is misspelling of the username or domain name. Internet e-mail addresses have to be spelled correctly for the mail to arrive at the correct destination.
Internet addresses consist of two parts, the username and the domain .
The domain is used to get the mail to the correct mail server, which will be the one the username subscribes to. The Essex mail server would file the mail under the username 'customer'. If the username 'customer' is misspelled, the server can't determine who the mail belongs to. So it bounces the message back to the sender. Just like using an incorrect address for US mail.