Originally Posted by
mherdeg
But with that rare exception this never, never, actually happens. You'll be asked to show your passport at check in at MCO and possibly at the gate at ORD, but there is no formal customs or immigration process for exiting the US.
Note that this is not US border control per se. It's because an airline can face large fines if one of their international passengers is denied entry upon arrival to the destination country. So the airlines tend to be rather to very careful about making sure all passenger paperwork is properly in order before people board an international flight.