Your Internet Consultant - The FAQs of Life Online
First, the host you're mailing to may not exist. The host (or site, the part of the e-mail address after the @ sign) must be listed in appropriate name servers. If the host you specify can't be found, your e-mail message has no destination and must be bounced back to you. When this happens, double-check your intended e-mail address and try to resend your message to the right place. Here's an example of a message bounced because there was no such site:
Date: Sun, 30 Jan 1994 20:40:38 -0800 To: waffle Subject: Returned mail: Host unknown ----- Transcript of session follows ----- 554 smith@nonexistent.com... 550 Host unknown (Authoritative answer from name server) ----- Recipients of this delivery ----- Bounced, cannot deliver: smith@nonexistent.com ----- Unsent message follows -----The other likely reason your e-mail may bounce is this: although the destination host has been verified, there is no user that answers to the name you specified. (The name is the part of the e-mail address that comes before the @ sign.) When this happens, double-check the name or username of your intended recipient and resend your message. Here's an example of a message that bounced because my cat doesn't have an account at apple.com:
From daemon Sun Jan 30 20:42:03 1994 Received: by bolero.rahul.net id AA25984 (5.67a8/IDA-1.5 for waffle); Sun, 30 Jan 1994 20:41:50 -0800 Date: Sun, 30 Jan 1994 20:41:50 -0800 From: Mail Delivery Subsystem <MAILER-DAEMON> Message-Id: <199401310441.AA25984@bolero.rahul.net> To: waffle Subject: Returned mail: User unknown ----- Transcript of session follows ----- While talking to apple.com: >>> RCPT To:<Kinsey_Michelle_Kitty@apple.com> <<< 550 <Kinsey_Michelle_Kitty@apple.com>... User unknown 550 Kinsey_Michelle_Kitty@apple.com... User unknown ----- Recipients of this delivery ----- Bounced, cannot deliver: Kinsey_Michelle_Kitty@apple.com ----- Unsent message follows -----