PTR record at the ISP

MXTool Test:

220 mail.becker.sc Microsoft ESMTP MAIL Service ready at Fri, 22 Oct 2010 09:48:32 +0200


Not an open relay.
 0 seconds - Good on Connection time
 6.037 seconds - Warning on Transaction time
 OK - 80.122.49.166 resolves to mail.chemo-phos.at
 Warning - Reverse DNS does not match SMTP Banner <--

Any chance your client is using Reverse-DNS checking now and that your
hostname isn't registered to that IP address (as PTR record at the ISP)?