Ultimately the keystrokes will be configurable.

The following are available now:

3-Pane Window as a whole

Keystroke	Command Executed
1	Focus on the list pane (servers, address book manager)
2	Focus on the items pane (mailbox, address book)
3	Focus on the preview pane
9	Cycle focus to previous pane
0	Cycle focus to next pane

Mailbox Pane (Single keys)

Keystroke	Command Executed
A, a	Edit:Select All
B, b	Message:Bounce
C, c	Copy Messages (does not delete after copy)
D, d	Message:Delete
E, e	Mailbox:Expunge
F, f	Message:Forward
I, i	Message:Flags:Important
L, l	Mailbox:Check Mail
M, m	Move Message (does delete after copy)
N, n	Select next new message
P, p	Select previous new message
R, r	Message:Reply
S, s	Message:Flags:Seen
W, w	Close currently active mailbox tab in 3-pane mailbox view
Y, y	Search the mailbox
Z, z	Zoom pane to full size or back to normal size
[	Select previous tab in 3-pane mailbox view
]	Select next tab in 3-pane mailbox view
<space>	Scroll down current message preview then go to next message
<tab>	Select next new message

Mailbox Pane (Double keys)

NB Second keystroke must occur within a short period of the first in order for the entire sequence to be recognised.

Keystroke 1	Keystroke 2	Command Executed
T,t (Only active when mailbox is sorted by Thread)	A, a	Select all messages in the threads of the currently selected messages
	S, s	Change all the seen flags for the messages in the threads of the currently selected messages
	I, i	Change all the important flags for the messages in the threads of the currently selected messages
	D, d	Change all the delete flags for the messages in the threads of the currently selected messages
	K, k	Select the first message in the next thread after the thread of the currently selected message
U, u	S, s	Match unseen messages
	I, i	Match important messages
	D, d	Match deleted messages
	T, t	Match messages sent today
	F, f	Match messages with the same From address as the currently selected message
	J, j	Match messages with the same Subject as the currently selected message
	A, a	Match messages with the same Date sent as the currently selected message
	U, u	Toggle match mode on and off

Message Window/Preview Pane

Keystroke	Command Executed
B, b	Message:Bounce
C, c	Copy Messages (does not delete after copy)
D, d	Message:Delete
F, f	Message:Forward
H, h	Toggle Full Header Display
I, i	Message:Flags:Important
M, m	Move Message (does delete after copy)
N, n	Read next message
P, p	Read previous message
R, r	Message:Reply
S, s	Message:Flags:Seen
V, v	Toggle Display of Parts
Z, z	Zoom pane to full size or back to normal size (preview pane only)
<space>	Scroll down current message then go to next message
<shift>+<space>	Scroll down message then go to previous message
<option/alt>+<space>	Scroll down message, delete, then go to next message
<tab>	Go to next new message
<backspace>, <delete>	Delete, then go to next message