mails are going in spam

I Am hosting a web application that needs the ability to send an email via SMTP. To that end, I created an SMTP virtual server within IIS 6.0(on windows server 2003) on the same webserver. Now the app sends emails perfectly. However, some domains drop the SMTP connection or refuse the email because the external IP address does not match the hostname with the receiver performs a Reverse DNS lookup.How I fix it. It is very urgent.

Best Regards,
Pawan