You cannot use username and password with vCenter SMTP authentication, only anonymous is allowed as of now.
For further information see the following kb article.
http://kb.vmware.com/kb/1004070
Verify that the mail server configuration allows for mail to be sent from the vCenter Server. The ability to send mail may be restricted so that anonymous email is not allowed or that specific user accounts are required to send mail. If this is the case a rule must be created to allow the vCenter Server to send email, as there is currently no way to configure SMTP authentication with vCenter Server.
This behaviour did not change in vCenter 5.1.