https://tools.ietf.org/html/rfc3030

This has been requested via email a couple of times

ametzler (12/18/16) It would be nice if swaks supported CHUNKING (BDAT instead of DATA). Exim has recently started supporting this (CHUNKING, but not BINARYMIME) and swaks is not yet able to test/use it.

mpurdon (6/30/14) - I'm wondering if support for Chunking could be added.