At first blush, the San Francisco airport website did not have a whole lot of helpful information on their “Passenger Services Listing” page:
Mailboxes are located curbside on the Departures/Ticketing Level at the end of all terminals.
At least it’s a point in the right direction. However, after poking around more on their site, I found the Interactive Map. A-ha! Click on that link and select Services > Mailboxes in the left column. The mailboxes are highlighted. Sweet!
I’m not a huge fan of things that change as you move your mouse around, but… if you look along the top of that map, you can select specific areas of the airport that you want to look at more closely (Terminal 1, Area B, for instance) and it will show you the mailbox closest to you, which is kind of cool.
It looks as though they are, indeed “at the end of all terminals.” The map helps you figure out which end!