Hiding private lan IP when using sendmail

[Gbenga Shobowale]

Hello All,
Can someone tell me how I can hide my IP from my ISP's box. Currently
my server relays mails to their server hence sendmail acts as a client
when connecting to their mail server and gets authenticated. All was
well until they blocked all IP hence when their server see my private
IP 192.168.0.128 for example the block and i get a message user
unknown.

Thanks