IPN sent to this URL have failed

Hello,
I've got a mail from Paypal today telling me to check the S2member IPN URL and suggesting to verify the server.
I've got a new member the 6th january without any problem so I don't know how to manage this. Do I need to contact my webhoster ?
I've got a mail from Paypal today telling me to check the S2member IPN URL and suggesting to verify the server.
I've got a new member the 6th january without any problem so I don't know how to manage this. Do I need to contact my webhoster ?