Since you would be using a single ip address for the domain name, its not possible to have two servers sharing the same ip, it would lead to an IP address conflict. There is no way of determining to which server the incomming mail would be directed.We had a afiled experiment when we tried a load balancing act with two exchange servers with the same IP so I guess it holds true in your case as well.
However you could consider accomplishing it this way.
1. Have Exchange 5.5 be your mail server. Create mail accounts on this server for people who need external mail facility. i.e. they can send as well as receive mail from people outside your organisation
2. Have a Linux server with Qmail or Send mail and qpopper configured within a local LAN and an internet connection to pop mail from exchange 5.5. Configure Sendmail to automatically attache the same domain name to the mail address. Users with accounts here can only send mail outside but would not be able to receive mail from outsidebecause anybody doing a 'reply to' or 'reply to all' would end up sending a mail to a mail address on your domain which does not exist.
This would give a LAN based bare bones mailing facility within your local group. With some users who also need external mail facilities, getting their mail popped out of exchage by qpopper and theu inturn popping it out of the local LINUX server.
Best regards,
Sridhar
