Get messages for the account which is bound to the identity then send the message when you click the toolbar button or the menu item.
If your SMTP sever authenticates using POP(IMAP) before SMTP, the extension is useful.
You can specify the account which is received in the extension's option from version 0.6.10. The SMTP server whose received account is set as "None" does not get messages before sending. I think you will not need the original send button by setting it appropriately :)
In order to use toolbar button, right-click on the toolbar and select "customize" then drag&drop the icon.
Please refer to the following link for supported locales:http://hogiblog.blogspot.com/2007/11/call-for-translation.html