Changes have been made in Git for this ticket:

updateAttendees() expects email addresses, not From: headers (Bug #10772).

  3 files changed, 7 insertions(+), 2 deletions(-)
http://git.horde.org/horde-git/-/commit/2c9fb69c7ca152ea5c34b655e09cbf8c7873fdf5

One possible fix is to change the check to a preg_match like this:

         if (is_null($sender) || !preg_match('/<?$attendee>?/', 
$sender_lcase)) {

Using Kronolith I invited a friend to a meeting.  The friend, using 
Horde, replied using Kronolith.  I received the reply via IMP and 
attempted to update his status in the meeting.  Kronolith refused, 
saying "The attendee hasn't been updated because the update was not 
sent from the attendee."

I tracked the cause down to a mismatch between the expected sender and 
the observed sender.  Because I had invited my friend using only his 
email address his response was rejected.  The problem line of code is 
kronolith/lib/Api.php line 1388 which reads:

                 if (is_null($sender) || $sender_lcase == $attendee) {

In this case, $sender_lcase was "ben klang <bklang@horde.org>" and 
$attendee was "bklang@horde.org"