There is two way to do branding
1)To specify by From and To Address
2) At the end in any email notification itself

Both the thing you can define in any email notification under "What it will contain" tab.
At the end of message create company specific signature. you can use html tag also for the same .
You can also create email template and use wherever you want for signature.
for example :



Also in From address you can specify the email address you want to use in From. For example, helpdesk@yourcompany.com.

For more detail you can see http://wiki.servicenow.com/index.php?title=Email_Notifications#Specifying_What_Content_the_Notification_Contains