not sure about the reason, but an example, where subject seems some sort of slavik variant (not greek anyway) in inbox, but displays normally in greek in message view… :
inbox view subject shown : тевмийг стгяинг
message view (correct) : Subject ΤΕΧΝΙΚΗ ΣΤΗΡΙΞΗ
raw header : Subject: =?iso-8859-7?B?1MXXzcnKxyDT1MfRyc7H?=
this one comes from : X-Mailer: Microsoft Windows Live Mail 16.4.3508.205, but seen the same attitude with messages originating from greek Outlook as well. (eg Microsoft Outlook 15.0)
we also had lots of encoding issues reported by clients (always in greek), that’s why we’re still using roundcube as primary webmail… there were also issues with user created folders in non-unicode greek encoding, that were unreadable in usermin, but i haven’t checked that lately…