bad is: Bad extension is related to Exchange Badmail File. You have chance to open bad files with Microsoft Exhange Server.
When email files which sent by Microsoft Exchange Server can`t reach the address, they are kept with BAD extension. Bad extension files include standard informations such as email address and receiver names. With bad fiels, there are .bdr and .bdp files. You can check free alternatives from links below.
Bad File format viewers or openers
Microsoft Exchange Server
Bad File reklated links.