Delist IP from Hotmail Block List

Hi,

  We have an an Exchange server 2010 R2. Around a month ago there was a virus in server which was spamming emails due to which our IP was blacklisted. After fixing the virus issue,  I had to request all Block list providers to delist our IP and after whcih I was able to successfully deliver email to other domains.

During period all is fine but hotmail is still not accepting emails from our domain.

 I requested my ISP to provide me new set of IP addresses hoping this would resolve the issue, but still hotmail is not accepting emails from our Domain with new IP addresses.

I have MX record and SPF record in place.

Below is the bounce back message I receive when sendng emails to hotmail accounts:

A problem occurred during the delivery of this message to this e-mail address. Try sending this message again. If the problem continues, please contact your helpdesk.

Kindly suggest me what steps should I take to make it work or remove IPs from hotmail blocklist.

Thanks

Regards
Atul Dangi