Problem

You send an email to someone and it bounces back with an error...

Reason

There are several reasons this can happen.

Most of which are answered by the error message, but it is important to read the error to understand what the problem is.

Solution

You may want to read what the error message says with regards to the problem.

For example:

Example of a bounce back message

----- The following addresses had permanent fatal errors ----- 
johndoe@gmail.edu 
(reason: 550 Host unknown)
(expanded from: <johndoe@engineering.ucsb.edu>) 
----- Transcript of session follows ----- 
550 5.1.2 johndoe@gmail.edu... Host unknown (Name server: gmail.edu: host not found) 
Action: failed 
Status: 5.1.2 
Remote-MTA: DNS; gmail.edu 
Diagnostic-Code: SMTP; 550 Host unknown 
Last-Attempt-Date: Tue, 16 Feb 2016 19:32:21 -0800

In this example, the error message says it could not deliver the message due to the fatal error and explaining the reason as "550 Host unknown".

The email was trying to send to johndoe@gmail.EDU. Since there is no gmail.EDU, the email bounced with an error saying 'Host unknown' as well as 'host not found'.

Here are the most common reasons mail will bounce back...

Email address is incorrect - Often times, it is a mis-spelled username or host.
User or Host does not exist - As the example shown above, gmail.edu does not exist as a host. Sometimes, it will also be a user does not exist for the host.
Recipient's mailbox is full - Some mail systems have a limit as to how much mail it can receive. Once a mailbox is full, it can state it is unable to receive the message for this reason.

The first two issues are fairly common and easy to fix by just re-verifying the email address you are sending to or contacting the person by another means to verify their address.

The last issue can only be handled by the recipient. They just need to make room on their mailbox or speak with their admin about their mail quota.

For more information on Email Error Messages, please refer to How to read Email Error Codes

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