We have added a workaround into Rebex Secure Mail which adds support for such malformed EML. The workaround was released as part of Rebex Secure Mail 2013R3 version on 3/12/2013.
Thank you for a nice catch, Gralin! This is already known as a bug which appeared on Samsung Galaxy Tab devices (e.g. see this forum).
The actual problem is that the device issues emails with Content-Type "multipart/relative" (which has no sense in MIME ) instead of the correct "multipart/related".