Summary

The mailto: markup doesn't render the email link properly when it has a leading underscore, eg. _email@example.com. It results in the link becoming inactive.

Cause

Confluence doesn't escape the _ (underscore) character because it's a reserved character. 

Solution

 We can achieve this using User Macro.

1. Go to    > General Configuration > User Macros
2. Choose Create a User Macro
3. Enter the details
   • Macro Name (e.g. leading-underscore-email)
   • Macro Title (e.g. Leading Underscore Email)
4. Set the Definition of User Macro to No macro body

5. In the template section enter the following:

   ## @param Email:title=Email Address|type=string|required=ture|desc=Enter email address 
   <a href="mailto:$!paramEmail">$!paramEmail</a>

6. Go to a page and add the newly created macro
   
7. Enter the email address
   
8. Click Insert
   
9. Save the page and verify the link works