For the links in the email body,try these:
A) For plain text email body
1.- To open a folder:
<file://C:\Location>
2. - To open a file
<file://C:\Location\testing_1.txt>
B) For HTML email body:
1.- To open a folder:
<a href="file://C:\Location" >A1</a>
2. - To open a file
<a href="file://C:\Location\testing_1.txt" >C3</a>
NOTE: you may also use "<file:\\<network_folder>\Location>" OR "<a href="file:\\<network_folder>\Location" >some text</a>".