I believe I have uncovered an Outlook bug. I've searched on the
Internet a bit but I didn't find much on this particular topic.

Basically it seems that Outlook (I'm using 2007) is overwriting
message flags at least when replying to messages. I have seen the
following behavior in the dovecot logs, first for only viewing a
message, second when replying to a message.

urfh UID STORE 65998 +FLAGS (\Seen)

3fxj IDLE DONE

ejm4 UID STORE 65998 FLAGS (\Seen \Answered)

The second STORE effectively wipes out any other flags on the message
and replaces them with \Seen & \Answered

I tried the same operation in Thunderbird and it performs properly.

Anyone else encountered this bug ? Anyone knows if it's fixed in 2010 ?