The CHUNKING extension (sometimes called BDAT) provides some features for SMTP to transmit very large binary messages. It's not used very much on its own, but rather together with BURL. It's required for Lemonade compliance. Together, the two extensions can be used to compose a message on the server, e.g. by joining a client-supplied header with a large attachment from another mail message (“forward without download”).
In case of questions, please write to firstname.lastname@example.org.