PV_Premier, I did an airside-search this a.m. for US Mail boxes at SFO T3 and came up empty. Also this was confirmed by experienced UA staff. As noted above, there are US Mail boxes at each SFO Terminal but none are airside.

If you have any chance to get to SMF early, I'd suggest marking your ballot and mailing there. (Must be a land-side box there.)

For those suggesting foreign mailing, if a Calif. ballot, the voted ballot has to be received by the County Registrar's office (or a county polling place) by Nov. 4, 8:00 p.m., for the next election, regardless of when the ballot was mailed or postmarked. I trained poll workers for many years and this requirement has been on the books a long time. So I'd say that the earlier and geographically closer the OP can mail to ballot's destination, the more likely it'll be counted.