Is it legal for an employer to access and read an employee's email account?

As title says.

I know there’s a case that the employee lost, but one of the factors considered was that his contract said that his email was subject to monitoring…

Company email belongs to the company. They can read them at their own discretion. If it’s a personal email account, that’s a different story.

Thanks for the response. Could you give me some references for this? I mean, which law says that? Thanks.

(I know for a fact that in other countries they can not do it, and they can be punished for doing it without a very good reason).

I’d like to see a reference too, but how could they force a look at your private e-mail if you are using your personal devices.

But it’s a general believed that all e-mail on the companies networks or equipment belong to the company.

I’d assume that that would also include personal web based accounts accessed on company equipment or even company provided hotspots.
Have a kill button ready to drop your account from your device or at least delete the password in an emergency if you really fear something.
Delete social media just before you have to show it to your boss if you are compelled to do so.