Procmail and error "is not a mailbox"

[rod...@gmail.com]

Hi,
I'm having a problem with fetchmail -> procmail -> mutt.
I'm fetching mail from an IMAP account, and using procmail to store and
sort it into local mbox mailboxes.
But after procmail finishes mutt won't open the mbox, saying it "is not
a mailbox".
In the mbox file which procmail generates, the "From" lines do not read
"From: ", but instead read "From ".
If I do formail -ds < mbox > mbox.new, then mutt will open the mbox.new
file. If I have fetchmail not invoke procmail, then mutt will also open
the mbox file.

My .fetchmailrc is:
poll $IMAP proto IMAP user "$USER" fetchall keep ssl folder "INBOX" mda
"/usr/bin/procmail -d %T"

My .procmailrc is simply:
:0
${MAILDIR}/Inbox

and the relevant portion of my .muttngrc is:
set mbox_type="mbox"
set spoolfile=~/mail/Inbox
set folder="~/mail"
set mbox="=Inbox"

I tried setting .fetchamilrc's mda to: "procmail -f - -d %T", and also
"formail -ds procmail",
but it still generates an mbox containing "From " instead of "From: ".
I also tried setting .procmailrc to filter through formail by using
this recipe, but it didn't solve the problem:
:0 fw
${MAILDIR}/Inbox
| /usr/bin/formail -b

I wasn't sure if formail could be invoked with different arguments to
solve this issue. Does anyone know what I should be doing to make this
work?

Thanks!