How could one read another's mail without knowing their ID

Question

for example in U.S some agencies, detect or tap some other's mail how could be it possible. how does mail travels

Answer 1

ACTUALLY IT IS NOT POSSIBLE

Answer 2

what has this got to do with baby names?

Answer 3

in the small print (like terms and conditions) there is probably something mentioned about government agencies etc. etc. in most countries the government is only allowed to do this if they have reasons and evidence that the email content may cause harm to others. it is like having to get a search warrant for someones house. they cant just hack into anyones email.