What could be causing the error below

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This report relates to a message you sent with the following header fields:

 Message-id: <242c666f1002050331k36980dfdjf1e4bc079e13aae@mail.gmail.com>
 Date: Fri, 05 Feb 2010 14:31:17 +0300
 From: MY_ADDRESS_HERE
 To: RECEIVERS_ADDRESS_HERE
 Subject: Test

Your message cannot be delivered to the following recipients:

 Recipient address: @barua.jambo.co.ke.lmtp:RECEIVERS_ADDRESS_HERE@lmtpcs-daemon
 Original address:RECEIVERS_ADDRESS_HERE
 Reason: LMTP transmission failure has occurred
 Diagnostic code: lmtp;550 5.2.2 Delivery failed: Over quota
 Remote system: dns;barua.jambo.co.ke (TCP|172.18.233.17|60116|172.18.233.14|225) (mx2.jambo.co.ke -- Server LMTP [Sun ONE Messaging Server 7.0-0.04 32bit [built Jun 20 2008]])


Original-recipient: RECEIVERS_ADDRESS_HERE
Final-recipient:
 rfc822;@barua.jambo.co.ke.lmtp:RECEIVERS_ADDRESS_HERE@lmtpcs-daemon
Action: failed
Status: 5.2.2 (LMTP transmission failure has occurred)
Remote-MTA: dns;barua.jambo.co.ke (TCP|172.18.233.17|60116|172.18.233.14|225)
 (mx2.jambo.co.ke -- Server LMTP [Sun ONE Messaging Server 7.0-0.04 32bit
 [built Jun 20 2008]])
Diagnostic-code: lmtp;550 5.2.2 Delivery failed: Over quota
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With Kind Regards
James Wachira
Nairobi .ke
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