You are going to have to digitally sign the project on each PC that the macro is used on to avoid the macro enabling issues and retain security -http://www.gmayor.com/create_and_employ_a_digital_cert.htm
While you can check if the To is a particular distribution list
e.g. If .To = "Bob"
I don't think you can check if To is any old distribution list without additional software - see
http://www.outlookcode.com/d/code/promptdl.htm