When a client is block systematically by Hotmail servers, he will have to subscribe to the Junk Mail Reporting Program.
Once the client fills the form and submits it, Hotmail will reply with a email with the subject line starting with SRX.
The client needs to send us that email, forward it or at least provide the SRX and the email address to which we need to send
a written confirmation that he, the client, is allowed exclusive rights over the IPs he has submitted via the form.
That confirmation needs to have the SRX code as the it's subject line (only that!).
If the client is still blocked (as seen in logs or headers) then he will have to contact Hotmail himself and address it with their staff.