I frequently read that in important correspondence one should use certified mail. But what good is this, really? All sending a letter this way will show is that I sent a letter and it was received. It does not verify the content of the letter, and the recipient could simply throw it away and deny what it was about. So, what is the point of certified mail for things like complaining to your apartment manager, requesting something with legal significance from your employer, etc?
Assume that this happened in a matter that goes to court. In civil court, there is no "innocent until/unless proven guilty". In civil court, the judge hears everyone's story, and decides which story is more likely to be true.
So I tell the judge "I sent a letter by registered mail; this is what was in the letter, and the post office reported to me that they delivered the mail, and someone signed for it". And you say "I never received a letter". The judge will believe me and the post office. You say "I received a letter and signed for it, but there was just a birthday card inside". Who does the judge believe? Does he or she believe that you received a letter with the contents I said and you are lying about it, or does she believe that I sent you an unsolicited birthday card by registered mail? Why would I do that? So they believe me.
Now if you said "I received a registered letter containing just a birthday court, so I immediately called my secretary and three other people in the office to see this and to verify there was nothing but the birthday card, and here they are as witnesses", then the court might start believing you.