Sending SMTP emails with MXO

What does SMTP 4.5.0 Error mean when sending emails?

Scenario:

I get a SMTP 4.5.0 Error when sending an email, What does it mean?
What does SMTP 4.5.0 Error mean when sending emails?

Solution:

SMTP 4.5.0 Error message is generated by MX Outbound when you have exceeded your quota.

The SMTP 450 error will also display the message "You have exceeded your MXTools quota. Contact http://www.mxtools.co.uk"