Sending a letter via “certified” mail, or an equivalent, means that the mail carrier will knock on the door and get a signature as evidence of delivery. But it isn’t always the best answer.
In court, it’s nice to have that signature if someone is saying that “I don’t know, I never got your letter.” So, it’s a thing that lawyers, and sometimes the laws, will often say you need to do. “Send it via certified mail.”
On the other hand, because the carrier has to knock and ask for a signature, certified mail is very easy to avoid. You just don’t answer the door, and the postal service can’t stick around forever, they give up after a few tries. (They need to deliver everyone else’s mail too, after all.) When this happens, the letter is just returned to sender, and a few weeks has probably passed.
So, if it’s more important that you get the demands in your letter to the other person, than it is you have strong evidence of delivery, “certified mail” is not the best way to go. Sometimes it’s a better move to disguise your letter as boring old mail, to make sure it gets seen.
In fact, this is why Juris DepositLetter works the way it does. We track the delivery process to the last possible moment, to provide updates, and a record… but our letter looks like boring old mail. Because the letter usually works, and you don’t have to go to court, its more important to get the letter through than it is to have a signature.