I remember the first time I came across a student who had no idea how to address the envelopes she needed to give her teachers along with her letter of recommendation packets. It was confirmation that I was much, much older than the students I was working with. Now it happens all the time, as many students have probably never had to write and mail a letter by hand.
While many schools have moved their letters of rec online, not all have done so. And addressing an envelope is probably still something students should know how to do whether or not you’re applying to schools where it would be helpful.
The US Postal Service has you covered. See page 12 of their guide, “A Customer’s Guide to Mailing.”