Well...most work contracts I've signed said something like "working here is not mandatory and we may need access to the mailbox (which we're providing to you for your work duties), such access cases are logged and externally audited. Sign here to agree, take that door to disagree." As long as this is agreed beforehand, I'm not aware of a European state banning it - this is somewhat different from "let's go digging around the computers out of curiosity". (I am in GMT+1, for the reference)
I've seen a situation where this was invoked - employee was fired for an unrelated issue, only kept some documentation in their inbox for whatever reason. Without such a provision, our options would have been a) legally questionable, b) up shit creek sans paddle.
In most of Europe what the employment contract says must be compared to local law - it varies greatly how many rights you are able to contract away in an employment contract.
E.g. while employment contracts in the UK are often fairly long, employment contracts in Norway can be as short as a couple of paragraphs, as almost all the terms are regulated and are costly and/or difficult to deviate from for most roles and most additional terms you might add will be null and void.
I've seen a situation where this was invoked - employee was fired for an unrelated issue, only kept some documentation in their inbox for whatever reason. Without such a provision, our options would have been a) legally questionable, b) up shit creek sans paddle.