Create a MIME message based on the given envelope
and bodies sections.
Parameters
envelope
An associative array of header fields. Valid keys are: "remail",
"return_path", "date", "from", "reply_to", "in_reply_to", "subject",
"to", "cc", "bcc" and "message_id", which set the respective message headers to the given string.
To set additional headers, the key "custom_headers" is supported, which expects
an array of those headers, e.g. ["User-Agent: My Mail Client"].
bodies
An indexed array of bodies. The first body is the main body of the message;
only if it has a type of TYPEMULTIPART, further bodies
are processed; these bodies constitute the bodies of the parts.