There are a couple of points here.
1. TS is brilliant with ISA of all flavours but that does not mean his is the only way to do things or the correct way in every circumstance. This is one area that I (personally) differ. If I want a front end box to filter mail such as spam filtrering etc then I place it in front of ISA rather than being on the ISA box itself
2. He suggests using the smtp service on the ISA for two reasons (I believe). Firstly you can use the message screener and second you can use it as a filtering service.
3 The recommended approach according to the MS courses that I attended and
teach use a publishing rule for smtp traffic whereby the inbound smtp traffic is forwarded to the IP address of the internal server (not the server name). For outbound smtp traffic, this is handled by a standard access rule in the firewall policy allowing smtp traffic to leave either to anywhere OR to the ip address of an external smarthost f that is the way you do things.
4. I then create a sesond publishing rule to allow access to the OWA & OMA services which in my case are on the same Exchange server.
5. To create the inbound publishing rule for Exchange, right-click the firewall policy and select new - publish a mail server selecting the smtp options within the wizard. You will be asked for the address of the internal smtp server that the traffic should be forwarded to. Hit the finish and apply the policy. For the outbound, most people cover this (by mistake) by allowing the outbound internet rule to allow all outbound protocols and of course, smtp is one of them. If you do not have this rule then simply create an access rule, allow, select smtp from internal to external, all users and done. If you really want to refine it then make a computer to represent the Exchange server and use this object in the from field rather than internal.