There's no direct way to get the epilogue.
Hopefully you're not running into messages that use the epilogue for anything useful.
RFC 2046 says:
NOTE: These "preamble" and "epilogue" areas are generally not used because of the lack of proper typing of these parts and the lack of clear semantics for handling these areas at gateways, particularly X.400 gateways. However, rather than leaving the preamble area blank, many MIME implementations have found this to be a convenient place to insert an explanatory note for recipients who read the message with pre-MIME software, since such notes will be ignored by MIME-compliant software.
While there's a reasonable use for the preamble, there's no suggestion of a reasonable
use for the epilogue.
What sort of real messages are you seeing that have data in the epilogue?
The MimeMultipart parser should stop reading when it reaches the terminating
boundary separator. You might be able to continue reading the InputStream
after that point to access the epilogue.