Mweb
Cannot send emails from IAFRICA email address

I can receive but cannot send any emails. Following is message displayed for one of the messages I tried to send :
Your message did not reach some or all of the intended recipients.

Subject: REPAIR NO 166472
Sent: 2013/04/03 11:08 AM

The following recipient (s) cannot be reached:

'Neels' on 2013/04/03 11:08 AM
Server error: '554 5.7.1 : Relay access denied'