5.6.1 on the format for sender name

The format string `%f' displays the return value of the function specified by wl-summary-from-function. If you use the function wl-summary-default-from (default), it displays sender name ordinarily, while displays the recipient names if the folder name matches with wl-summary-showto-folder-regexp and the sender is yourself. If the value of wl-use-petname is Non-nil, it uses petname to display.

For example, to display recipient names for the message in `+backup' if its sender is yourself, set up as follows.

(setq wl-summary-showto-folder-regexp "^\\+backup$")