You can use the Rmail summary buffer to do almost anything you can do in the Rmail buffer itself. In fact, once you have a summary buffer, there's no need to switch back to the Rmail buffer.
You can select and display various messages in the Rmail buffer, from the summary buffer, just by moving point in the summary buffer to different lines. It doesn't matter what Emacs command you use to move point; whichever line point is on at the end of the command, that message is selected in the Rmail buffer.
Almost all Rmail commands work in the summary buffer as well as in the Rmail buffer. Thus, d in the summary buffer deletes the current message, u undeletes, and x expunges. o and C-o output the current message to a file; r starts a reply to it. You can scroll the current message while remaining in the summary buffer using SPC and DEL.
The Rmail commands to move between messages also work in the summary buffer, but with a twist: they move through the set of messages included in the summary. They also ensure the Rmail buffer appears on the screen (unlike cursor motion commands, which update the contents of the Rmail buffer but don't display it in a window unless it already appears). Here is a list of these commands:
Deletion, undeletion, and getting new mail, and even selection of a
different message all update the summary buffer when you do them in the
Rmail buffer. If the variable
nil, these actions also bring the summary buffer back onto
When you are finished using the summary, type w
rmail-summary-wipe) to delete the summary buffer's window. You
can also exit Rmail while in the summary: q
rmail-summary-quit) deletes the summary window, then exits from
Rmail by saving the Rmail file and switching to another buffer.