Stepan,
Firstly, why would you want the out side world knowing your internal IP address of your SMTP server? This defeats the objective of having a firewall. You can not change what you are asking for as all your internal IP traffic is being translated to one public IP address and the return traffic is being port address translated (PAT) back to your internal network.
You have a good setup so I would not change anything unless it is causing you problems, also from a security point, if you want to make your pix invisible to any port scanners from the outside world, you can issue command: (in config mode) - icmp deny any outside, you can check if you have any ports open by going to www.grc.com and try out Shields Up. This program will probe your firewall for any open ports and will display a report of the results. Try it before you issue the icmp deny command suggested above and then try it after applying it. The grc shields up is a secure probe and I've used it quite often.
Let me know if this helps.
Jay
