cat single.msg |sed -e '/^$/ q'

and

   cat single.msg |sed -e '1,/^$/ d'

but I don't always get perfect results from those sed commands.

However, it would seem necessary to first split the text
into legitimate mail messages from "<lf>From " to "<lf>From ".

In doing this, I would also have to eliminate any garbage text
(i.e. that text that isn't part of the header or body.
Much of this in the said binary mailbox (Entourage's Database)
is 4- or 5-character lines e.g. 9CTID or FLRC and I'm not sure
how those can be properly identified since a paragraph could end
with something like
this.

Thus, I'm trying to convert a binary mailbox into a
standard sendmail mailbox.  Thanks for any ideas, thoughts
or other help.

Ooops!  It's harder than I thought since I first assumed
that Microsoft preserved the header line "<lf>From "
but they didn't and it's not clear what they use to
delineate the beginning and end of a mail message.

Maybe you can use the "export" or "save" commands of the mail program
that created the file.

--
A furore Normanorum libera nos, O Domine!
        [From the fury of the nor{*filter*} deliver us, O Lord!]
                -- Medieval prayer