[imp] Invalid "From" header with Forward->Redirect

[Leonardo Boiko]

Hi,

When I try Forward->Redirect, something somewhere prepends a From line
without a colon and with a date (an envelope from?), like this:

    From user at example.com  Thu Mar  3 10:09:12 2016

I uploaded a full example email here:
http://namakajiri.net/misc/redirected.eml.txt
This is a message sent from user at example.com to user at example.com, and
then redirected from inbox to a different user.

This of course creates an invalid email.

I don't know whether it is Horde doing this, or some misconfiguration
in our Postfixes or Spamassassin or what.

Any suggestions in how do we debug this?