This is Europe.
There _is_ no unpaid overtime!
I've worked for UK, Spanish and German companies and and sure there is, it's done all the time. Often when you work longer hours in the day and do the odd bit of work at the weekend.
If people check their mail during vacation, they are working, and they have to be paid and their vacation is still due an they can sue the company when they leave (or not) to get payment for the missed holidays or weekends.
IANAL but I believe that only counts if they specifically ask you to check your email. If they don't and you go ahead anyway, then you won't get paid for it.
I always leave my blackberry at home when I go on holiday.