I believe you can. From what it sounds like, your form has a drop down box, which when value X in said drop down box is selected, you want a text box in the form to populate with a corresponding email address based on the selection of value X in drop down menu.
If this is what you are trying to accomplish, this is rather simple to do, I did this some time back so it's a bit hazy but bascially you will need to do this through sharepoint designer. Create a workflow that poplutates textbox1 from the sharepoint list in question, based on the value of dropdownbox1.
It is largely just a conditional operator
(
If the value of field1 on form1 is X
Then the value of field2 on form1 populates with Y data from sharepointlist1
)
Does that make sense to you?