Hi,
since we moved our mailserver to new hardware (including new IP address) all mails to messagelabs.com servers are being blocked or rather it looks like the packages are being dropped since I don't even get any response (telnet to port 25 from other servers respond with the proper welcome string).
According to http://ipremoval.sms.symantec.com/lookup/ the server IP does not have a bad reputation (The IP address you submitted, 136.243.174.235, does not have a negative reputation and therefore cannot be submitted for investigation.) Could you please check on your side and let me know what exactly the issue is since we do not event get a rejection notice to follow up upon.
Thank you.