If you have setup a mail client on your computer and get an error similar to the following when attempting to send mail:
Your message did not reach some or all of the intended recipients.
Subject: Test Email
Sent: 18/03/2014 10:52
The following recipient(s) cannot be reached:
'[recipient email]' on 18/03/2014 10:52
Server error: '553 sorry, that address is not in my list of allowed recipients (#5.7.1)'
Then please ensure that you have configured outbound authentication within your email client.
When you send an email our system requires you to provide your username and password to verify that you can send email. Without this information being passed you will encounter the above error.
Please follow the setup guides for your email client on this support site and ensure that the program is setup in the same way.