Thanks nukkumatti.
I tried the approach you suggested but I couldn't get it to work. It's very possible that it's because I am not knowledgable enough with coding. I also tried several other approaches that I read online on various sites, and I couldn't get any of them to work either. However, I found an approach in the InfoPath 2003 SDK documentation that seemed to work great. Using the Source property, I was able to populate the Subject field of the e-mail message with node values from my form.
I also tried to understand the purpose of each line, and after testing the code I was able to eliminate several lines.
function CreateMailEnvelope::OnClick(eventObj)
{
var oEnvelope = Application.ActiveWindow.MailEnvelope;
oEnvelope.To = "DL name or e-mail address A; DL name or e-mail address B";
oEnvelope.CC = "DL name or e-mail address C";
oEnvelope.Visible = true;
oEnvelope.Subject = eventObj.Source.selectSingleNode("//my:ForReview").text + eventObj.Source.selectSingleNode("//my:DocumentTitle").text;
var objView;
objView = XDocument.Mail;
XDocument.View.SwitchView("mail");
}