It's a crime to open someone else's mail and generally speaking the post office does a pretty good job of reliable delivery. Even if an address is a bit wrong/corrupted, it can likely be delivered just from the name and the zipcode.

Email is a lot harder. The older SMTP standard sends emails unencrypted so there's a possibility of a MITM reading the email. But also addresses if you get them wrong can end up in the wrong hands. For example, if someone sends an email to cogman10, I'll get it, but if they go to cogman1O I won't get it. A lot of the nuance of how secure and when it's secure gets erased by auditors to just "email is insecure".