The CHUNKING service extension was originally described in RFC 1830 published August 1995 and is now a proposed standard defined in RFC 3030 published December 2000.
RFC 821 (circa 1982) defined the DATA command to send messages. The DATA command required email content to be sent as a series of (short) lines of US-ASCII text.
The vast majority of email traffic today is transported using TCP. TCP places no restrictions of the format of data byond a stream of octets.
The CHUNKING extension introduces the BDAT command to send messages. This command lifts the restrictions placed on email by the old DATA command. With BDAT, any octet stream is fine. BDAT is an excellent match with TCP.
Several of the top email providers support this extension, including Google (gmail.com) and Microsoft (outlook.com, hotmail.com).
Not many of the major FLOSS MTA software programs support this feature. Good news from Qsmtp is they have support. Also, Exim now has support.
I would like to encourage universal adoption of the CHUNKING extension.