Balsa's main window has three major areas. To the left is the mailbox list. This displays all the mailboxes Balsa knows about; it will scan for ones in your local mail directory. (This means Balsa works well with procmail.) It will also list IMAP or POP3 mailboxes that you have configured.
At the top-right is a notebook view of all the currently open mailboxes. You can browse messages, manipulate them, and switch between open mailboxes here.
At the bottom-right of Balsa's main window is the message view. This displays the message selected in the message listing. You can save attachments, view attachments, and read incoming email here.
The mailbox list in the main window lists all of your mailboxes. Balsa will automatically detect your local mailboxes by searching for mailboxes under your local mail directory (probably ~/mail). This lets Balsa coexist with pine and procmail well.
Your mailboxes will be displayed as a tree view, with mailboxes in subdirectories of your local mail directory initially hidden but accessible by expanding the tree. POP3 and IMAP mailboxes will also be displayed as being at the top level of the tree.
Useful mailbox list menu items are:
Open the mailbox selected in the mailbox list.
Close the mailbox selected in the mailbox list.
Close the mailbox shown in the message index.
Commit changes to the mailbox shown in the mailbox list.
Commit changes to the mailbox shown in the message index.
Create a new local mailbox.
Edit the properties of the mailbox selected in the mailbox list.
Remove the currently selected maibox -- delete its data as well as Balsa's record of it.
You can assign keybindings to these commands, and all menu items in GNOME, by clicking and releasing on the menu, selecting the item, and pressing a key combination.
The message index displays the messages in the current folder, and lets you choose which folder's messages to view. Click on the notebook tabs, or double-click on a folder in the mailbox list to bring it to the front.
From the message index you can do a lot. Clicking on a message will cause it to be displayed in the message view. Right-clicking on it will give a popup menu letting you reply to the message (or forward it, etc.) and move it to another folder (using the Transfer menu item). Double-clicking on a message will open it in a separate window.
There are some useful hotkeys for the message index.
Go to the next message
Go to the previous message
Delete the current message
Undelete the current message
Reply to the current message
Reply-to-all the current message
Forward the current message
There are also menu items for many of the same functions.
Close the currently displayed mailbox.
Commits your changes the currently displayed mailbox.
Replies to the current message.
Replies-to-all the current message.
Forwards to the current message.
Continues the current message -- opens it unmodified in a composition window. This works for any message.
Deletes to the current message.
Undeletes to the current message.
The message view displays the message that is selected in message list. It will attempt to display images in the message. Balsa does not understand HTML. (Someday, maybe.) Attachments will be shown as blue links that look like "--APPLICATION--". Clicking on the link will let you save (but not view) it.
You are not restricted to that pane. Double-click on a message in the message index to open it in a separate window.
Balsa's message view is a little deficient. It will not line-wrap messages and cannot handle MIME data other than images. Hopefully this will be fixed one day.