In the address book, accessible through Alt-A, the Addressbook button, or through the tool menu, you store e-mail addresses of the people and places you want to write more often.
Adding an address to the address book is quite easy too. Rightclick the folder or group you want to add the address to, select "new address" and fill in the dialog that comes up. You can also directly click the Add button in the address book window, and the dialog comes up. In this case the address is moved into the folder/group that is selected at the moment of adding.
Deleting an address is equally simple. Find the address, click it once, and press the Delete button.
The address book, like the mail folders, can be expanded into an entire tree of sections. For this you right-click on the folder where you want to add a new folder, and select "new folder". Then you can enter a descriptive name for the folder, click Ok, and your folder is created.
In this same way you can create a new group in a folder. Right-click on the folder, select "new group", enter a name for it, and that's it.
Open the address-book through one of the available ways (toolbar, shortcut key). Finding an e-mail address: enter part of the start of the name or the address you are looking for and press the TAB key. A dropdown list will appear with the selections that have been found to match. Select one and press enter. After that you can use the To:, Cc: or Bcc: buttons to add the address to an e-mail, if you are writing one. Otherwise these buttons are greyed out and not functional.