I'm using Avira AntiVir (www.free-av.de) and it detects stunnel 4.20 as virus APPL/Stunnel.A . I do not know what to do against it... as you can see in this list, stunnel was added to the signatures Database of AntiVir: http://www.avira.de/en/threats/section/vdfhistory/vdf_no/6.37.00.53/6.37.00....
Maybe one of the devs can check back with avira....
best regards, daniel