How to send mass emails to users in salesforce

In the below sample code "lstPortalUser" is a list of partner users.Now I need to sent them an email with different body.

1. List<Messaging.SingleEmailMessage> mails = new List<Messaging.SingleEmailMessage>();  
2. for(User portalUser :lstPortalUser)  
3. {     
4.    Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();  
5.    string body = 'Hi '+ portalUser.LastName;    
6.    mail.setSubject('Test Subject');  
7.    mail.setTargetObjectId(portalUser.Id);  
8.    mail.setSaveAsActivity(false);  
9.    mail.setHtmlBody(body);  
10.    mails.add(mail);  
11. }  
12. Messaging.sendEmail(mails);