COS451 email address bouncing?

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Delivery has failed to these recipients or distribution lists: [email]cos451@osprey.unisa.ac.za[/email] The format of the recipient's e-mail address isn't valid. A valid address looks like this: [email]username@contoso.com[/email]. Microsoft Exchange will not try to redeliver this message for you. Please check the e-mail address and try sending the message again, or provide the following diagnostic text to your system administrator. The following organization rejected your message: osprey.unisa.ac.za.