I've read either here or some other oracle forum that if you have outlook "server", I can't remember its correct name, running on the Web server then you can automatically send the emails out without the user having to click send. This would be for when you run your forms over the internet / intranet.
Otherwise from what I've heard (i've not worked with forms for a while) you can use the OLE extention to Oracle forms to open a mail in outlook and populate it (although the user must click send). The client must have outlook installed on there machine though (and may need it running at the time)
So that's how I'd do it if you wanted it to be initiated by the client. The alternative is to do it from the actual DB server. If your on 8.1.6 then you have a handy DBMS package called DBMS_SMTP that will do emails for you, There is an excellent FAQ on it in this forum.
Does that cover it??
HTH,