Here is a map of the message array that contains all the information about a message. A single part message is handled the same as a multipart message, except in the "ENTITIES" part, there would only be one entry (0).

$message     ["HEADER"]                    (array)
                ["TO"]                     (array)
                ["CC"]                     (array)
                ["SUBJECT"]                (string)
                ["FROM"]                   (string)
                ["REPLYTO"]                (string)
                ["MAILER"]                 (string)
                ["TYPE0"]                  (string)
                ["TYPE1"]                  (string)
                ["BOUNDARY"]               (string)
                ["CHARSET"]                (string)
                ["MIME"]                   (boolean)
                ["ENCODING"]               (string)

             ["ENTITIES"]                  (array)
                [0]                        (array)
                   ["TYPE0"]               (string)
                   ["TYPE1"]               (string)
                   ["CHARSET"]             (string)
                   ["BOUNDARY"]            (string)
                   ["PRIORITY"]            (integer)
                   ["BODY"]                (array of strings)

                [1]                        (array)
                   ["TYPE0"]               (string)
                   ["TYPE1"]               (string)
                   ["CHARSET"]             (string)
                   ["BOUNDARY"]            (string)
                   ["PRIORITY"]            (integer)
                   ["BODY"]                (array of strings)
             .
             .
             .


Example message:
----------------------------------------------------------------------
$message     ["HEADER"]
                  ["TO"]                     "luke@usa.om.org,"
                                             "nathan@usa.om.org"
                  ["CC"]                     "matt@usa.om.org"
                  ["SUBJECT"]                "hey there.. just testing"
                  ["FROM"]                   "typist@usa.om.org"
                  ["REPLYTO"]                ""
                  ["MAILER"]                 "SquirrelMail v0.0.1"
                  ["TYPE0"]                  "multipart"
                  ["TYPE1"]                  "alternative"
                  ["BOUNDARY"]               "--blkjoaiu2093ojv0q9"
                  ["CHARSET"]                "us-ascii"
                  ["MIME"]                   true
                  ["ENCODING"]               "us-ascii"

             ["ENTITIES"]
                  [0]
                     ["TYPE0"]               "text"
                     ["TYPE1"]               "plain"
                     ["CHARSET"]             "us-ascii"
                     ["BOUNDARY"]            ""
                     ["PRIORITY"]            10
                     ["BODY"]                "This is just a test to see"
                                             "how this will handle a message"
                                             "for this example"

                 [1]
                     ["TYPE0"]               "text"
                     ["TYPE1"]               "html"
                     ["CHARSET"]             "us-ascii"
                     ["BOUNDARY"]            ""
                     ["PRIORITY"]            20
                     ["BODY"]                "<B>This is just a test to see</B><BR>"
                                             "<FONT FACE="Arial,Helvetica">how this will handle a message"
                                             "for this example</FONT>"