For vCenter v6.0, if you would like to enable SMTP authentication settings you could do so actually in the vCenter Server Settings -> Advance Settings :
KEY : VALUE
mail.sender : The alert email address e.g. alert@mydomain.com
mail.smtp.username : The valid SMTP account name e.g. david@mydomain.com
mail.smtp.password : The valid SMTP account password
mail.smtp.port : The SMTP receiving port e.g. 25
mail.smtp.server : The SMTP host e.g. mail.mydomain.com
Hope this help. Not too sure if this is valid for v5.5 but for sure this is working for my current v6.0 environment that use plain SMTP authentication.