From the RFC : Multiple parts are indicated by parenthesis nesting. Instead of a body type as the first element of the parenthesized list, there is a sequence of one or more nested body structures. For example, a two part message consisting of a text and a BASE64-encoded text attachment can have a body structure of: (("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 1152 23)("TEXT" "PLAIN" ("CHARSET" "US-ASCII" "NAME" "cc.diff") "<960723163407.20117h@xxxxxxxxxxxxxxxxxx>" "Compiler diff" "BASE64" 4554 73) "MIXED")