If you enter [[Special:Emailuser]] without specifying the target user, you get the following error message:
"You have not specified a target page or user on which to perform this function."
It'd be great to have an input mask instead, where you can just enter the user name and get the page Special:Emailuser/username (or Special:Emailuser?target=username).
I think, this is also how it's done in Special:Contributions
Version: unspecified
Severity: enhancement