I insert a link “[User Name] (mailto: username@domain.com)” and get link “mailto: //username@domain.com”, with “://” - which is not correct.
What’s wrong?
Just had the exact same problem. 
That is a Markdown issue. It’s not valid Markdown. You can use
<username@domain.com>
or simply html
<a href="mailto:username@domain.com">User Name</a>
instead.
Actually this does look like an issue on our end… Please create an issue here and i’ll get it fixed asap: https://github.com/getgrav/grav/issues
Fixed with this commit: https://github.com/getgrav/grav/commit/04116b3a37b7327d1938010e0d55411e4d9b89f8