I was trying to recomment another SMTP plugin to one of my friends, the one I'm currently using: http://wordpress.org/extend/plugins/wp-smtp/
Then I realized it has been removed from this site. Instead I could find yours that has a really similar code. By looking at the dates it was clear that the other was first.
I know everyhing is free an opensource here, but that's more of a reason to indicate if your stuff is based on the code of someone else's work.
Of course it's just a note and I appreciate that you took over the maintenance and keep the plugin updated for recent WP versions, thanks for that.