To detail Eva's answer some more.
When you have created your lead. You can go under the white box with all information regarding your lead and click on the text "Send a mail".
It is located in between the white box and your followers and the line says "Send a mail" or "Make a note".
If you click send a mail then it opens up a little text box. If you want to have more options then press on the square on the right top just above the text box. This will let you use templates which have been pre installed or created by you. And gives you a text editor with more options.
One thing to keep in mind when using this function is that it will send this mail to all of your leads followers and the e-mail adresses you enter (they have to be specified in your system already or else OpenERP will ask you to create them)
The only way to send a completely random e-mail is like Eva already mentioned. You go to Settings -> Under the section header Technical click on E-mail and then messages.
When you pressed the create button you will get a form which allows you to send random e-mails to anybody you want (as long as they are specified or you create them on the go).
Make sure to add in the FROM field, because this will be the e-mail adress it will show in your recipients Sender section.
I hope this makes it clearer. If not please let me know, I can make you a small tutorial with pictures.
I now understand what you want to do. Is it possible that you create a sub context in your dashboard with a direct link to the send msg under techinical setting? This way they don't have to dig in your settings and they can have a fast link in one click when they are at their dashboard(starting page). Is that a viable solution for you? If so, but you don't know how to do this, just leave a comment and I will make a detailed description for this.
Hoped I helped you with this.