Sometimes when Mail is running it appears to do absolutely nothing, yet at the same time CPU is running full pelt and your network is being used. What is probably happening is Mail is sending or receiving a large number of emails. The best way to check what is happening is with the Mail Activity monitor. It is quick to access and very useful.
To access the Mail Activity box click the small button in the bottom left hand corner. Its the second one in.
This small box gives you loads of options. For example you can see which emails are in the queue, the size and where they are going. As well as this you also get the opportunity to stop an email being downloaded if you know it is two large or one you don’t want.
Its only a quick tip but for many people, including myself, you may have forgotten it is there. It is certainly useful for diagnosing problems.