Why am I getting this "553 sorry, that domain isn't allowed to be relayed thru this MTA (#5.7.1)" error when I try to send emails?

This error is a result of your email client (i.e. Outlook, Mac Mail, Thunderbird, etc.) attempting to send email without first authenticating with our mail server.  In an effort to prevent some of the potential spam we require our customers to authenticate first.

If you are receiving the 553 error, don't worry, it is easy to fix.  Below I are instructions for various email clients and email client versions.  Simply choose yours and follow the included instructions.  I your email client is not included please let us know and we will add yours to our list.

Outlook 2003:
http://www.authsmtp.com/outlook-2003/default-port.html

Outlook 2007:
http://www.authsmtp.com/outlook-2007/default-port.html

Outlook 2010 & 2013:
http://www.tradebooster.com/web-hosting-articles/how-to-enable-smtp-authentication-in-outlook-2010/

Windows 8 Mail App:
http://www.authsmtp.com/windows-8-mail-app/

Window Live Mail:
http://blog.arvixe.com/how-to-turn-on-smtp-authentication-on-windows-live-mail/

Outlook Express:
https://support.exabytes.com.my/KB/a1944/how-to-enable-smtp-authentication-for-microsoft-outlook.aspx?KBSearchID=113458

Exchange Server:
http://kb.mimecast.com/Mimecast_Knowledge_Base/Getting_Started_with_Mimecast/Implementation/Mimecast_Connect/Routing_Outbound/Configure_SMTP_Authentication_on_Exchange

Mac Outlook 2011:
http://www.authsmtp.com/outlook-2011-mac/alternate-port.html

Mac Mail:
https://customers.machighway.com/knowledgebase.php?action=displayarticle&id=381

Thunderbird:
https://support.mozillamessaging.com/en-US/kb/unable-to-authenticate-to-smtp-server

Entourage:
http://support.znet.com/faqs/entourage-smtp-auth.html

iPhone/iPad:
Settings   ->   Mail,Contacts,Calenders   ->   Select your desired email account   ->   SMTP   ->   Select your desired email account again, and then fill out the following:
Hostname = mail.yourdomain.com
Username = your full email address
Password = your email password
Use SSL = OFF
Authentication = Password
Server Port = 587

Android:
Open the "Email" application
Click the settings button (3 dots on most devices)
Select your account
Scroll to "Outgoing Settings" and select
If it's not there, enter the following:
username = full e-mail address
password = e-mail password
Make sure the box for "Require Sign-in" is check marked
Push done and send a test message