I've got a couple clients, most are running 10, some are 9. If I write up something in FM that uses the SMTP options, would the sites running 9 need to upgrade both the client and server apps to get the functionality or just the clients?
If you are sending an email through a send mail script step initiated by an end user using client then yes you will have compatibility issues. However, if instead you have something like your script create a bunch of record and then a server side script send emails based on those records, then you can just use FMS10.
IOW, if the sending action is done on the server, then its okay.