As Ralf says different boards will show up on different ports, my Windows 7 box seems to pick up some unique id so "identical" boards will show up on different Com's but each board gets the same one each time. Linux will give a board the lowest available port when it is plugged in.
In any case have you considered plugging the Uno into the host port of the yun so it can send messages to tell the linino part of the yun to send an email? You would still have the Leonardo side of the yun available for I/O and processing.
You would have to learn some Linux scripting to put all this together but isn't learning the point?