The jumbled letters and digits you see are
>uuencoded< content. In fact this is contained within any email that includes binary attachments (images, programs, zip files, etc). Email can only contain ASCII/Unicode characters, so binary stuff has to be
uuencoded into ASCII/Unicode before transmission and then
uudecoded at the other end. Most modern email programs do this automatically, transparent to the end user.
In the case of this particular message, it probably has malformed headers, due to a defect in the way the email was constucted by the spammers' software. A common problem, since spammers are always morons. The malformation prevents your email program from decoding the content, for which you should be grateful, actually!!!