Did you receive a message which obviously is spam? Simply mark the message(s) you want to report as spam and select 'Mark As Spam' on the toolbar. Marking message as ´Spam´ will allow us to train our spam filters and eventually increase the possibility to detect more spam messages in the future.
The marked messages are removed to the 'Spam' folder. Of course, if you mistakenly report a message as spam, you can undo this: Go to the 'Spam' folder, select the respective message and mark it as 'Not Spam'.