If you read the documentation in snippets/eform/docs/eform.htm you'll see there's already a parameter for doing just that...
&mailselector (Optional)
Sets the name of the form field to use as a selector to select a single email from the comma (,) delimited emails assigned the &toparameter. This selector field will act like a numeric index to select an email. It will start at 1 for the first email and ends at N for the last email in the list.
For example: &to =`sales@me.com,support@me.com,billing@me.com` &mailselector=`topic`
On the web form the topic field is actually a dropdown menu. when the user selects a topic from the list the value 1,2 or 3 will be sent to eForm which will then be used to select one of the three emails assigned to the &toparameter. This email address will be the address used to send the email to.