Exchange 2010 - Message submission rate has been exceeded?

[rush_tech]

I'm receiving a error in moodle when trying to send email via our mail server which is below

 

Failed to send an email from the user with id '1642' to the user with id '2360' due to the following error: 
"The following From address failed: mmail@domainname : MAIL FROM command failed,421,4.4.2 Message submission rate for this client has exceeded the configured limit ".

I've checked the receive connector messageRateLimit that moodle uses and it's unlimited and still I receive the errors, can anyone suggest where else to look?

[DarkCloud]

What you really do need to do is post something about cats! Then you'll get a reply.