Hi Devang,
At this time, SmarterMail does not support CIDR format for IP ranges. To enter an IP range, use a dash (-) to separate the IPs. For example, you'd need to enter 65.55.88.0-65.55.88.255 in the IP field instead.
While looking into this, we did also discover a bug that prevented a gateway's single IP being changed to a range. The fix for that issue is coming in the next minor release of SmarterMail.
Andrea Free
SmarterTools Inc.
877-357-6278
www.smartertools.com