Safari (desktop and mobile) includes a behavior similar to pop-up blockers where it tries to detect if the triggering of a mailto link has a corresponding user gesture. If it can’t detect the gesture, it blocks the email composition.
It appears that the pushing of a Coda button with the “Open Hyperlink” action and a “mailto” URL causes this block to occur. You can see this issue being discussed more generally on Stack Overflow
Any suggestions on a fix? HTML links work, but aren’t nearly as snazzy looking.