the Topic itself asks the Question. What does
cat emailsource | formail -I "From "
exactly? Does it simply remove all duplicate Froms? The Manpage is a little bit unclear to me.
Removes all the "From " headers from the message.
The "From " line is the delimiter for a unix mailbox. Notice, it doesn't contain a ":" like the "From:" header
Essentially UNIX mbox format keeps all your mails in one file. This is the polar opposite to something like Maildir which treats every mail as a separate file. Because multiple mails are in one file, they needed a delimiter and chose "From " at the top of the message
http://www.jwz.org/doc/content-length.html
This would normally be done to convert a UNIX mbox mail so that it can be used elsewhere by a client or format that doesn't support it.
e.g.
formail -s < /var/mail/phil | formail -I "From " # ...