You will need to add an undocumented Site Configuratin Parammeter.
This parameter became available in QCv10 Patch 12.
Below are the instructions from the QCv10 Patch 12 Readme.doc
**NOTE**
1. Test on a test system before implementing in producion.
2. The instructions state a restart of the Quality Center service is required.

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QCV10 README.DOC
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SMTP Authentication
Defect ID: 232925
Problem: SMTP authentication is now supported.

The following SMTP settings are supported by Quality Center:

* Basic authentication

* Use of TLS (Transport Layer Security) which uses base64 encryption of the communication between Quality Center and the SMTP server.

* Only authenticated users can send emails.

To enable the feature:

1. Add the SMTP_AUTHENTICATION site parameter, and set the parameter value to 'Y'.
2. In Site Administration, select Site Configuration > Settings > Set Mail Protocol. Configure the following fields:
a. User name and password of a mail administrator user that has permissions to connect to the SMTP server.
b. Email address of the SMTP admin user.
3. Restart the Quality Center service.
Note: if the SMTP_AUTHENTICATION parameter is set to 'N' or deleted, the above fields are hidden again, and Quality Center will not use authentication to login. You must restart the Quality Center service to apply the changes.