It's easy .....
1. checkout wheather On clicking Compose mail takes you to a page where you can enter the text.
2.Check wheather it has a TO,CC,BCC to enter email address.
3.Check wheather TO,CC,BCC accepts valid email address,if it does how many mail address can be entered.
4.Checkout if you can delete/edit ,copy, paste & format the text content.
5.Can sendmail or cancel mail.
U can go on........... with testcase I hope u have some good idea now.