thanks for the quick response - at least not we know what the problem is :)

Not quite. OL doesn't communicate with the IMAP server at all. Horde 
determines that an incoming EAS message is a reply/forward to an 
existing email (it does this in a number of ways, depending on what 
data is available from the client). Then it saves the reply 
information in the mail log configured in IMP - this is where the time 
data comes from. While Horde also sets a flag on the IMAP server as 
well, this is not involved with the message reply state in EAS.
Horde is sending the required WBXML to OL to indicate the current 
reply state of the email. OL is deciding to ignore this data:

2014-05-09T11:27:03-04:00 DEBUG: [21971] O        <POOMMAIL2:LastVerbExecuted>
2014-05-09T11:27:03-04:00 DEBUG: [21971] O         1
2014-05-09T11:27:03-04:00 DEBUG: [21971] O         
</POOMMAIL2:LastVerbExecuted>
2014-05-09T11:27:03-04:00 DEBUG: [21971] O         
<POOMMAIL2:LastVerbExecutionTime>
2014-05-09T11:27:03-04:00 DEBUG: [21971] O         2014-05-09T15:26:51.000Z
2014-05-09T11:27:03-04:00 DEBUG: [21971] O         
</POOMMAIL2:LastVerbExecutionTime>

A google search for something like "Outlook 2013 message reply state" 
should give you a number of (non-horde) hits describing this problem.

TLDR; We cannot do anything about it.

EMails answered in Outlook will have their answered flag synced back 
to the IMAP server (so horde webmail and other clients will see the 
flag set) but it doesn't work in the opposite direction: Outlook will 
not notice via EAS when emails have been answered in webmail / other 
clients.