Message format

This is known to be along the lines of

CMD <TrID> <parameters> <payload size>\n
<header name>: <header value>\n
[...more headers...]\n
\n
<payload body>

The payload body often itself constitutes a headers-and-body construct.

<TrID> is a transaction ID; outlook.com uses numbers for this. If it's set to something nonzero in a message sent by the client, the server will acknowledge the message by means of a response--possibly one that is otherwise a noop--that has the same transaction ID. If the server sends an unsolicited message, the transaction ID is zero.