%D The day of the date field of the message (zero padded).
%W The weekday name of the date field of the message (zero padded).
%h The hour of the date field of the message (zero padded).
+%l The number in the mailing list.
%m The minute of the date field of the message (zero padded).
%[ An open bracket. If the message thread is linked,
it is replaced with '<'.
%f The from: field string of the message.
%s The subject: field string of the message.
%S The size of the message (if available).
+%t The branch of the thread.
+%@ `@' only if the first MIME part is multipart/mixed.
%~ If the previous spec is not zero-length, replaced with ' '.
+%# mailing list information (`(' ML-name [ ` ' ML-number ] `)')
If the format string contains the specifiers %( and %), the text between
them will have the specified number of columns.