There have been some unusual CSS problems we've noticed since 12.5.5409. Most of the issues I've heard of are with Chrome, but here's something to try.
In IIS Manager change the SmarterMail Application Pool from Integrated to Classic and see if that solves the problem. It solves the Chrome problem and might resolve your issue.
Please report back if it solves the problem.
Thanks,
-Joe
Thanks,
-Joe