Lucky you, you get to learn about the double eval hack! You'll need to create an email submit connection -- more details on that are available at Office Online, here. In the To field, select the formula editor (fx) and in the formula window, you'll enter something very like this:
xdMath:Eval(xdMath:Eval(my:group1/my:field1, 'concat(., ";")'), "..")
You need to change my:group1/my:field1 to point to the repeating field that your multi-select list box is bound to. You can use the Insert field or group button to do this.
For more information on this hack, there is a great blog post available
here.