We have setup an SBS server 2003 that is acting as mail server (it is responsible for sending and receiving emails directly) with an static IP address. In 90% of the times the emails are reaching the recipients but there are some domains for which when an email is being sent from this server does not reach the recipient. However, we are able to send an email to the same recipient through gmail or another non sbs server. We have contacted the ISP who is hosting our recipient's email to check whether our IP is blacklisted on their server and the response was negative. Any idea why this is happening? is our IP address tagged incorrectly?