Alright, so I was curious myself, and I found the exact reason why...
This explanation will be for the technically-minded people that are familiar with some of these forum workings, so if you're not interested, then just skim through. So osu! forums is formed out of PHPBB, which, when writing your email, goes through a PHP function that checks for email validation. This runs the content in the email field through what we call a regular expression, which attempts to parse the expression and sees if it notices what seems like an email.
Here's the expression.The expression
(((?:[\w\!\#$\%\&\'\*\+\-\/\=\?\^\`{\|\}\~]+\.)*(?:[\w\!\#$\%\'\*\+\-\/\=\?\^\`{\|\}\~]|&)+)@((((([a-z0-9]{1}[a-z0-9\-]{0,62}[a-z0-9]{1})|[a-z])\.)+[a-z]{2,63})|(\d{1,3}\.){3}\d{1,3}(\:\d{1,5})?))\w+seems pretty fool-proof, but actually, there is a check in there that requires the ending domain to be at least three characters for it to work.
Here's what happens when we run the expression through RegExr.
So even as mail.ru is a valid email provider, the validation expression fails to notice it.
I recommend sending an email to
accounts@ppy.sh requesting your account to have its email changed to this manually, and see how they can help you out. I also recommend to take down all the screenshots that contain your email (or blacken or mark them out) for safekeeping too.