Backport r58545.
diff --git a/Doc/lib/libmailbox.tex b/Doc/lib/libmailbox.tex
index 24765c8..b52304f 100644
--- a/Doc/lib/libmailbox.tex
+++ b/Doc/lib/libmailbox.tex
@@ -699,7 +699,7 @@
subdirectory) or "cur" (if the message should be stored in the \file{cur}
subdirectory). \note{A message is typically moved from \file{new} to \file{cur}
after its mailbox has been accessed, whether or not the message is has been
-read. A message \code{msg} has been read if \code{"S" not in msg.get_flags()}
+read. A message \code{msg} has been read if \code{"S" in msg.get_flags()}
is \code{True}.}
\end{methoddesc}