There is another question here that asks something similar, but apparently it refers to the US, where secrecy of correspondence doesn't seem to exist, at least according to this article on Wikipedia: https://en.wikipedia.org/wiki/Secrecy_of_correspondence
In several European countries that principle is included though. According to that article, it probably started as a principle that applied to paper mail and phone calls, but has been extended to other forms of communication on the internet (email, etc.).
So, if you are a system administrator who owns a server which is used to provide an email service, can you ever read some of the emails sent by a user to another user? If you are a sysadmin who owns a forum or a social network that allows its members to send private messages to each other, can you ever read some of those messages? For example, somebody might report a TOS violation to you, or even a possible crime, so for example you might be told that John Doe is sending malware via private messages or emails, and you might want to look into it and end up reading some of them. This might sound like a legitimate interest of the sysadmin, if it wasn't for that principle of secrecy of correspondence. I might be wrong, but it sounds like a sysadmin would need some kind of warrant in such cases.
Jurisdictions: the European countries that have secrecy of correspondence in the constitution. If you want to add some info about the US by comparison, that'll be interesting as well.