The problem is that it needs too many logons since I have a number of accounts to monitor. I know it is an Exchange server administration problem, not related to JavaMail, but I really hope you can give me some ideas. We're not really experts in Exchange here, so we can't really tell you if there's a good way to do this. However, I think you're on the right track. In general, sessions don't make a difference here.
Sessions are mostly just a way to organize properties and configuration information.
You might see a performance increase by using multiple connections to your mail server. You'll probably need to experiment to see how much this helps with your particular mail server. Yes, there is a difference between the two. When the session object is created, it reads the system properties and caches the information. Subsequent calls to set system properties will not be part of any previously created Session objects.