diff --git a/Doc/lib/libmailbox.tex b/Doc/lib/libmailbox.tex
index 947c2d3..767b678 100644
--- a/Doc/lib/libmailbox.tex
+++ b/Doc/lib/libmailbox.tex
@@ -18,6 +18,10 @@
 object.  The default is the \class{rfc822.Message} class (see the
 \refmodule{rfc822} module -- and the note below).
 
+\note{For reasons of this module's internal implementation, you will probably
+want to open the \var{fp} object in binary mode.  This is especially important
+on Windows.}
+
 For maximum portability, messages in a \UNIX-style mailbox are
 separated by any line that begins exactly with the string \code{'From
 '} (note the trailing space) if preceded by exactly two newlines.