It's not, not in Europe, not anywhere. Saying it a couple of times doesnt make it true. Your boss can read your emails and most likely already does. This is exactly the same with any internal messaging and it's perfectly legal in the workspace.
[1] https://www.coe.int/en/web/human-rights-rule-of-law/-/echr-m...
From the PDF press release linked on the COE site:
"The national courts [have not] carried out a sufficient assessment of whether there had been legitimate reasons to justify monitoring Mr Bărbulescu’s ommunications."
"Neither of the national courts had sufficiently examined whether the aim pursued by the employer could have been achieved by less intrusive methods than accessing the contents of Mr Bărbulescu’s communications."
In other words: You need a legitimate reason for accessing your employees' communication and you need to consider less intrusive ways of achieving your aim first.
So your boss is not allowed to just read your mail whenever he likes. Or maybe he is, if you're working in the US. But European courts, especially the international ones, are very strict about privacy and protecting personal rights.