There are a number of key equivalents in the application which were designed to ease repetitive tasks or provide additional functionality unobtrusively. The following is a partial attempt at documenting these. (Reminder: the "Command" key has the small clover and Apple icon on it; the Option key is also known as the Alt key):
In this context...
do this...
in order to...
Message or address list
shift-down arrow or shift-up arrow
Extend a selection by one item
type any key (A-Z)
Select an item whose name starts by...
Command-click
Select discontiguous items
Shift-command-click (OS 9)
Shift-click (OS X)Select a range of items
Command-option-click outside of a selected item
Move around the lists in "hand" mode
Narrow the list header for dates
Display dates in short format
Use up-down arrows when in mail body or msg list
View previous/next message
Space bar
Scroll down current message, then view next unread message
Shift-space bar
Scroll up current message, then view previous unread message
Click on the "Eye" icon, near the horizontal scroll bar
Show the "View only" toolbar (narrows down a list)
Option-click on a message
See a message's full headers
Drag column headers
Move column positions around
Option-drag column headers
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Message window displaying a received message
control <- or ->
go to previous / next message
control shift <- or ->
go to previous / next unread message
control option <- or ->
delete current message and go to previous / next message
control shift option <- or ->
delete current message and go to previous / next unread message
Note: You may use command [ or ] instead control <- or ->, but variants with shift and option keys might not work with non US keyboards
Recipient field of an outgoing message
Command- up or down- arrow
Toggle between To, CC and BCC
Text body
Option-tab
Move cursor out up to the subject field
Command up- or down- arrow
Move cursor to top or bottom of text
Command left- or right- arrow
Move cursor to left or right end of line
Option left- or right- arrow
Move cursor to previous or next word
shift-arrow
Extend the selection
Address book
Enter
Edit the selected recipients
Command-Enter
add the selected items as "To:" recipients of the current message (or of a new message if none is open)
Other notes
Command-S
Saves the current message as draft; in Setup windows, saves the current settings
PowerMail startup
Command-Option
Open the PowerMail First Aid dialog
Shift-Command-Option
Open PowerMail using another user environment, or to create a new user environment