munpack is a handy program for unpacking MIME-encoded mail messages. It will decode most of the MIME-encoding types out there, but people keep adding new ones, so your mileage may vary.
Some mail programs unpack MIME-encoded messages automatically, others do not. To use munpack, either pipe your encoded message to it, or run munpack on a file that contains the encoded message.
See the man page for munpack for more details.